Organic geochemical and micropaleontological analyses of surface
sediments collected in the southern Drake Passage and the Bransfield Strait, Western
Antarctic Peninsula, enable a proxy-based reconstruction of recent sea ice
conditions in this climate-sensitive area. We study the distribution of the
sea ice biomarker IPSO25, and biomarkers of open marine environments
such as more unsaturated highly branched isoprenoid alkenes and
phytosterols. Comparison of the sedimentary distribution of these biomarker
lipids with sea ice data obtained from satellite observations and
diatom-based sea ice estimates provide for an evaluation of the suitability
of these biomarkers to reflect recent sea surface conditions. The
distribution of IPSO25 supports earlier suggestions that the source
diatom seems to be common in near-coastal environments characterized by
annually recurring sea ice cover, while the distribution of the other
biomarkers is highly variable. Offsets between sea ice estimates deduced
from the abundance of biomarkers and satellite-based sea ice data are
attributed to the different time intervals recorded within the sediments and
the instrumental records from the study area, which experienced rapid
environmental changes during the past 100 years. To distinguish areas
characterized by permanently ice-free conditions, seasonal sea ice cover and
extended sea ice cover, we apply the concept of the PIP25 index from
the Arctic Ocean to our data and introduce the term PIPSO25 as a
potential sea ice proxy. While the trends in PIPSO25 are generally
consistent with satellite sea ice data and winter sea ice concentrations in
the study area estimated by diatom transfer functions, more studies on the
environmental significance of IPSO25 as a Southern Ocean sea ice proxy
are needed before this biomarker can be applied for semi-quantitative sea
ice reconstructions.
Introduction
In the last century, the Western Antarctic Peninsula (WAP) has undergone a
rapid warming of the atmosphere of 3.7±1∘C, which exceeds
several times the average global warming
(Pachauri et al., 2014; Vaughan et
al., 2003). Simultaneously, a reduction in sea ice coverage
(Parkinson and Cavalieri, 2012), a shortening of the
sea ice season (Parkinson, 2002) and a decreasing sea ice
extent of ∼ 4 % per decade to 10 % per decade (Liu et al., 2004) are recorded
in the adjacent Bellingshausen Sea. The loss of seasonal sea ice and
increased melt water fluxes impact the formation of deep and intermediate
waters, the ocean–atmosphere exchange of gases and heat, the primary
production, and higher trophic levels (Arrigo
et al., 1997; Mendes et al., 2013; Morrison et al., 2015; Orsi et al., 2002;
Rintoul, 2007). Since the start of satellite-based sea ice observations,
however, a slight increase in total Antarctic sea ice extent has been
documented, which contrasts with the significant decrease in sea ice in Western
Antarctica, especially around the WAP (Hobbs et al.,
2016).
For an improved understanding of the oceanic and atmospheric feedback
mechanisms associated with the observed changes in sea ice coverage,
reconstructions of past sea ice conditions in climate-sensitive areas such
as the WAP are of increasing importance. A common approach for sea ice
reconstructions in the Southern Ocean is based on the investigation of sea-ice-associated diatom assemblages preserved in marine sediments
(Bárcena
et al., 1998; Gersonde and Zielinski, 2000; Heroy et al., 2008; Leventer,
1998; Minzoni et al., 2015). By means of transfer functions, this approach
can provide quantitative estimates of paleo-sea-ice coverage (Crosta
et al., 1998; Esper and Gersonde, 2014a). The application of diatoms for
paleoenvironmental studies, however, can be limited by the selective
dissolution of biogenic opal frustules (Burckle
and Cooke, 1983; Esper and Gersonde, 2014b) in the photic zone
(Ragueneau
et al., 2000) and in surface sediments (Leventer, 1998). As
an alternative or additional approach to diatom studies, Massé et al. (2011) proposed the use of a specific biomarker lipid –
a diunsaturated highly branched isoprenoid alkene (HBI C25:2, Fig. 1a)
– for Southern Ocean sea ice reconstructions. The HBI diene was first
described by Nichols et al. (1988) from sea ice diatoms.
δ13C isotopic analyses of the HBI diene suggest a sea ice origin for
this molecule (Sinninghe
Damsté et al., 2007; Massé et al., 2011) and this is further
corroborated by the identification of the sea ice diatom Berkeleya adeliensis as a producer of
this HBI diene (Belt et al.,
2016). Berkeleya adeliensis is associated with Antarctic landfast ice and the underlying
so-called platelet ice (Riaux-Gobin and Poulin,
2004). In a survey of surface sediments collected from proximal sites around
Antarctica, Belt et al. (2016) note a widespread
sedimentary occurrence of the HBI diene and – by analogy with the Arctic
HBI monoene termed IP25 (Belt et al., 2007) –
proposed the term IPSO25 (ice proxy for the Southern Ocean with 25
carbon atoms) as a new name for this biomarker.
The molecular structures of (a) IPSO25, (b) the HBI Z-triene
and (c) the HBI E-triene.
In previous studies, an HBI triene (HBI C25:3; Fig. 1b–c) found in
polar and sub-polar phytoplankton samples (Massé et
al., 2011) has been considered alongside IPSO25, and the ratio of
IPSO25 to this HBI triene has hence been interpreted as a measure for
the relative contribution of organic matter derived from sea ice algae
versus open water phytoplankton (Massé
et al., 2011; Collins et al., 2013; Etourneau et al., 2013; Barbara et al.,
2013, 2016).
Collins et al. (2013) further suggested that the HBI
triene might reflect phytoplankton productivity in marginal ice zones (MIZs)
and, based on the observation of elevated HBI triene concentrations in East
Antarctic MIZ surface waters, this has been strengthened by Smik et al. (2016a). Known source
organisms of HBI trienes (Fig. 1 shows molecular structures of both the E-
and Z-isomer) are, for example, Rhizosolenia and Pleurosigma diatom species (Belt
et al., 2000, 2017). In the subpolar North Atlantic, the HBI Z-triene has
been used to further modify the so-called PIP25 index
(Smik et al., 2016b) – an approach for semi-quantitative
sea ice estimates. Initially, PIP25 was based on the employment of
phytoplankton-derived sterols, such as brassicasterol
(24-methylcholesta-5,22E-dien-3β-ol) and dinosterol (4α,23,24-trimethyl-5α-cholest-22E-en-3β-ol)
(Kanazawa et al., 1971; Volkman, 2003), to serve
as open-water counterparts, while IP25 reflects the occurrence of
former sea ice cover (Belt et
al., 2007; Müller et al., 2009, 2011). Consideration of these different
types of biomarkers helps to discriminate between ice-free and permanently
ice-covered ocean conditions, both resulting in a lack of IP25 and
IPSO25, respectively (for further details see
Belt, 2018 and Belt and Müller, 2013). Uncertainties in the
source specificity of brassicasterol (Volkman, 1986) and its
identification in Arctic sea ice samples, however, require caution when
pairing this sterol with a sea ice biomarker lipid for Arctic sea ice
reconstructions (Belt et al., 2013). In
this context, we note that Belt et al. (2018) reported that brassicasterol is not
evident in the IPSO25-producing sea ice diatom Berkeleya adeliensis. While the
applicability of HBIs (and sterols) to reconstruct past sea ice conditions
has been thoroughly investigated in the Arctic Ocean (Belt,
2018; Stein et al., 2012; Xiao et al., 2015), only two studies document the
distribution of HBIs in Southern Ocean surface sediments (Belt et al., 2016;
Massé et al., 2011). The circum-Antarctic data set published by Belt et
al. (2016), however, reports neither HBI triene nor sterol abundances. Significantly more studies
so far focused on the use of IPSO25 and the HBI Z-triene for paleo-sea-ice reconstructions, and these records are commonly compared to
micropaleontological diatom analyses (e.g.
Barbara et al., 2013; Collins et al., 2013; Denis et al., 2010).
Here, we provide the first overview of the distribution of IPSO25, HBI
trienes, brassicasterol and dinosterol in surface sediments from the
permanently ice-free ocean in the area from the Drake Passage to the seasonal sea ice
inhabited area of the Bransfield Strait at the northern WAP. Sea ice
estimates based on biomarkers are compared to sea ice concentrations derived
from diatom transfer functions and satellite-derived data on the recent sea
ice conditions in the study area. We further introduce and discuss the
so-called PIPSO25 index (phytoplankton-IPSO25 index), which,
following the PIP25 approach in the Arctic Ocean
(Müller et al., 2011), may serve as
a further indicator of past Southern Ocean sea ice cover.
Oceanographic setting
The study area includes the southern Drake Passage and the Bransfield Strait
located between the South Shetland Islands and the northern tip of the WAP
(Fig. 2a and b). The oceanographic setting in the Drake Passage is dominated
by the Antarctic Circumpolar Current (ACC) and several oceanic fronts
showing large geostrophic water mass flows and subduction and upwelling of
water masses (Orsi et al., 1995).
The Antarctic Polar Front (APF) divides relatively warm subantarctic waters
from the cold and salty Antarctic waters, while the southern Antarctic
Circumpolar Current Front (SACCF) is often associated with the maximum sea ice
extent (Kim and Orsi, 2014). The current
system in the Bransfield Strait is relatively complex, and the mixture of
water masses is not yet well understood (Moffat and Meredith,
2018; Sangrà et al., 2011). A branch of the ACC enters the Bransfield
Strait in the west as the Bransfield Current, carrying transitional waters
under the influence of the Bellingshausen Sea (Transitional Bellingshausen
Sea Water, TBW). The TBW is characterized by a well-stratified, fresh and
warm water mass with summer sea surface temperatures (SSTs) above
0 ∘C. Below the shallow TBW, a narrow tongue of circumpolar deep
water (CDW) flows along the slope of the South Shetland Islands
(Sangrà et al., 2011). In the
eastern part, transitional water from the Weddell Sea (Transitional Weddell
Sea Water, TWW) enters the Bransfield Strait through the Antarctic Sound and
from the Antarctic Peninsula (AP). This water mass corresponds to the
Antarctic Coastal Current (Collares
et al., 2018; Thompson et al., 2009). The TWW is significantly colder
(summer SST < 0 ∘C) and saltier due to extended sea ice
formation in the Weddell Sea Gyre. The two water masses are separated at the
sea surface by the Peninsula Front characterized by a TBW anticyclonic eddy
system (Sangrà et al., 2011). While
the TWW occupies the deep water column of the Bransfield Strait
(Sangrà et al., 2011), it joins the
surface TBW in the southwestern Bransfield Strait
(Collares et al., 2018).
(a) Oceanographic setting of the study area
(modified after
Hofmann et al., 1996; Sangrà et al., 2011); ACC: Antarctic
Circumpolar Current, TBW: Transitional Bellingshausen Water, TWW: Transitional Weddell Water, APF: Antarctic Polar Front, SACCF:
Southern Antarctic Circumpolar Current Front and PF: Peninsula Front,
and the maximum winter sea ice extent (after
Cárdenas et al., 2018). (b) The bathymetric map of the study area with
locations of all stations; AP: Antarctic Peninsula, AS: Antarctic
Sound, BS: Bransfield Strait and SSI: South Shetland Islands. A
detailed station map at the South Shetland Islands is integrated.
The overview maps were created with QGIS 3.0 from 2018 and the bathymetry was
taken from GEBCO_14 from 2015.
Due to high concentrations of dissolved iron on the shelf (Klunder
et al., 2014), the area around the WAP is characterized by a high primary
production with high vertical export fluxes during early summer associated
with the formation of fast-sinking mineral aggregates and fecal pellets
(Kim et
al., 2004; Wefer et al., 1988). The Peninsula Front divides the Bransfield
Strait into two biogeographic regimes of high chlorophyll and diatom
abundance in the TBW and low chlorophyll values and a predominance of
nanoplankton in the TWW (Gonçalves-Araujo et al.,
2015), which is also reflected in the geochemistry of surface sediments
(Cárdenas et al., 2018).
Materials and methodsSediment samples and radiocarbon dating
In total, 26 surface sediment samples obtained by multicorers and box corers
during the RV Polarstern cruise PS97 (Lamy, 2016) were analysed (Fig. 2,
Table 1). All samples were stored frozen and in glass vials. The composition
of the sediments ranges from foraminiferal mud in the Drake Passage to
diatomaceous mud with varying amounts of ice rafted debris in the Bransfield
Strait (Lamy, 2016).
Coordinates of sample stations with water depth, concentrations of
IPSO25, HBI Z- and E-trienes, brassicasterol, and dinosterol normalized
to TOC; δ13C values for IPSO25; and values of sea ice
indices PIPSO25 based on the HBI Z- and E-trienes, brassicasterol, and
dinosterol. Concentrations below the detection limit are expressed as 0. The
PIPSO25 could not be calculated where IPSO25 and the phytoplankton
marker is absent (blank fields).
14C radiocarbon dating of two samples from the PS97 cruise and one from
the Polarstern cruise ANT-VI/2 (Fütterer, 1988) was conducted using
the mini carbon dating system (MICADAS) at the Alfred Wegener Institute
(AWI) in Bremerhaven, Germany, following the method of Wacker et al. (2010). The 14C ages were calibrated to calendar
years before present (cal BP) using the Calib 7.1 software
(Stuiver et al., 2019) with an estimated reservoir age
of 1178 years, derived from the six closest reference points listed in the
Marine Reservoir Correction Database (http://calib.org, last access: 7 November 2018).
Organic geochemical analyses
For biomarker analyses, sediments were freeze-dried and homogenized using an
agate mortar. After freeze-drying, samples were stored frozen to avoid
degradation. The extraction, purification and quantification of HBIs and
sterols follow the analytical protocol applied by the international
community of researchers performing HBI and sterol-based sea ice
reconstructions (Belt
et al., 2013, 2014; Stein et al., 2012). Prior to extraction, internal
standards 7-hexylnonadecane (7-HND) and 5α-androstan-3β-ol
were added to the sediments. For the ultrasonic extraction (15 min), a
mixture of CH2Cl2:MeOH (v/v2:1; 6 mL) was added to the sediment.
After centrifugation (2500 rpm for 1 min), the organic solvent layer was
decanted. The ultrasonic extraction step was repeated twice. From the
combined total organic extract, apolar hydrocarbons were separated via open column chromatography (SiO2) using hexane (5 mL). Sterols were eluted
with ethylacetate–hexane (v/v20:80; 8 mL). HBIs were analysed using an
Agilent 7890B gas chromatography (30 m DB 1MS column, 0.25 mm diameter,
0.250 µm film thickness, oven temperature 60 ∘C for 3 min,
increase to 325 ∘C within 23 min, holding 325 ∘C for 16 min) coupled to an Agilent 5977B mass spectrometer (MSD, 70 eV constant
ionization potential, ion source temperature 230 ∘C). Sterols were
first silylated (200 µL BSTFA; 60 ∘C; 2 h; Belt et al., 2013; Brault and
Simoneit, 1988; Fahl and Stein, 2012) and then analysed on the same
instrument using a different oven temperature programme (60 ∘C for 2 min, increase to 150 ∘C within 6 min, increase to 325 ∘C
within 56 min 40 s). As recommended by Belt (2018), the identification of
IPSO25 and HBI trienes is based on comparison of their mass spectra
with published mass spectra (Belt,
2018; Belt et al., 2000; see Fig. S1 in the Supplement). Regarding the
potential sulfurization of IPSO25 we examined the GC-MS
chromatogram and mass spectra of each sample for the occurrence of the HBI
C25 sulfide (Sinninghe
Damsté et al., 2007). The C25 HBI thiane was absent from all
samples. For the quantification, manually integrated peak areas of the
molecular ions of the HBIs in relation to the fragment ion m/z 266 of 7-HND
were used. Instrumental response factors are determined by means of an
external standard sediment from the Lancaster Sound, Canada. The HBI
concentrations in this sediment are known and a set of calibration series
was applied to determine the different response factors of the HBI molecular
ions (m/z 346; m/z 348) and the fragment ion of 7-HND (m/z 266) (Fig. S2; Belt, 2018; Fahl and Stein, 2012). The identification of sterols was
based on comparison of their retention times and mass spectra with those of
reference compounds run on the same instrument. Comparison of peak areas of
individual analytes and the internal standard was used for sterol
quantification. The error determined by duplicate GC-MS measurements was
below 0.7 %. The detection limit for HBIs and sterols was 0.5 ng g-1
sediment. Absolute concentrations of HBIs and sterols were normalized to
total organic carbon (TOC) content (for TOC data see
Cárdenas et al., 2018).
The herein-presented phytoplankton-IPSO25 index (PIPSO25) is
calculated using the same formula as for the PIP25 index following
Müller et al. (2011):
PIPSO25=IPSO25IPSO25+c×phytoplankton marker.
The balance factor c (c= mean IPSO25/mean phytoplankton biomarker)
is applied to account for the high offsets in the magnitude of IPSO25
and sterol concentrations (see Belt and
Müller, 2013, Müller et al., 2011, and Smik et al., 2016b for details
and a discussion of the c factor). Since the concentrations of IPSO25
and both HBI trienes are in the same range, the c factor has been set to 1
(following Smik et al., 2016b). For the calculation of
the sterol-based PIPSO25 index using brassicasterol and dinosterol the
applied c factors are 0.0048 and 0.0137, respectively.
Stable carbon isotope composition of IPSO25, requiring a minimum of 50 ng carbon, was successfully determined on five samples using GC-irm-MS. The
ThermoFisher Scientific Trace GC was equipped with a 30 m Restek Rxi-5 ms
column (0.25 mm diameter, 0.25 µm film thickness) and coupled to a
Finnigan MAT 252 isotope ratio mass spectrometer via a modified GC/C interface. Combustion of compounds was done under continuous flow in ceramic
tubes filled with Ni wires at 1000 ∘C under an oxygen trickle
flow. The same GC programme as for the HBI identification was used. The
calibration was done by comparison to a CO2 monitoring gas. The values
of δ13C are expressed in per mille (‰)
against Vienna Pee Dee Belemnite (VPDB), and the mean standard deviation was
<0.9 ‰. An external standard mixture was
measured every six runs, achieving a long-term mean standard deviation of
0.2 ‰ and an average accuracy of <0.1 ‰. Stable isotopic composition of neither HBI trienes
nor sterols could be determined due to coeluting compounds.
Diatoms
Details of the standard technique of diatom sample preparation were
developed in the micropaleontological laboratory at the AWI in Bremerhaven, Germany. The preparation included a
treatment of the sediment samples with hydrogen peroxide and concentrated
hydrochloric acid to remove organic and calcareous remains. After washing
the samples several times with purified water, the water was removed and the
diatoms were embedded on permanent mounts for counting
(see detailed description by Gersonde and Zielinski,
2000). The respective diatom counting was carried out according to Schrader
and Gersonde (1978). On average, 400 to 600 diatom valves were counted in each slide using a Zeiss Axioplan 2 at ×1000
magnification. In general preservation state of the diatom assemblages was
moderate to good in the Bransfield Strait and decreased towards the Drake
Passage where it is moderate to poor.
Diatoms were identified to species or species group level and if possible to
forma or variety level. The taxonomy follows primarily Hasle and Syvertsen (1996), Zielinski and Gersonde (1997), and Armand and Zielinski (2001). Following Zielinski and Gersonde (1997) and Zielinski et al. (1998) we combined some taxa into the following groups.
The Thalassionema nitzschioides group combines T. nitzschioides var. lanceolata and T. nitzschioides var. capitulata, two varieties with a gradual transition
of features between them and no significantly different ecological response.
The species Fragilariopsis curta and Fragilariopsis cylindrus were combined as the F. curta group, taking into account their similar
relationship to sea ice and temperature (Armand et al., 2005;
Zielinski and Gersonde, 1997). Furthermore, the Thalassiosira gracilis group comprises T. gracilis var.
gracilis and T. gracilis var. expecta because the characteristic patterns in these varieties are often
transitional, which hampers distinct identification.
Although the two varieties, Eucampia antarctica var. recta and E. antarctica var. antarctica, display different
biogeographical distribution (Fryxell and Prasad, 1990), they
were combined to the E. antarctica group. This group was not included in the transfer
function (TF) as it shows no relationship to either sea ice or temperature
variation (Esper
and Gersonde, 2014a, b). Besides the E. antarctica group, we also discarded diatoms
assembled as Chaetoceros spp. group from the TF-based reconstructions, following
Zielinski et al. (1998) and Esper and Gersonde (2014a).
This group combines mainly resting spores of a diatom genus with a
ubiquitous distribution pattern that cannot be identified to species level
due to the lack of morphological features during light microscopic
inspection. Therefore, different ecological demands of individual taxa
cannot be distinguished.
For estimating winter sea ice (WSI) concentrations we applied the marine
diatom TF MAT-D274/28/4an, comprising 274 reference samples from surface
sediments in the western Indian, the Atlantic and the Pacific sectors of the
Southern Ocean, with 28 diatom taxa and taxa groups, and an average of 4
analogues (Esper
and Gersonde, 2014a). The WSI estimates refer to September sea-ice
concentrations averaged over a time period from 1981 to 2010 at each surface
sediment site (National
Oceanic and Atmospheric Administration, NOAA; Reynolds et al., 2002, 2007).
The reference data set is suitable for our approach as it uses a
1∘ by 1∘ grid, representing a higher resolution than
previously used and results in a root mean squared error of prediction
(RMSEP) of 5.52 % (Esper
and Gersonde, 2014a). We defined 15 % concentration as threshold for
maximum sea-ice expansion following the approach of Zwally et al. (2002)
for the presence or absence of sea ice, and 40 % concentration
representing the average sea-ice edge (Gersonde et al.,
2005; Gloersen et al., 1993). MAT calculations were carried out with the
statistical computing software R (R Core Team, 2012) using the
additional packages Vegan (Oksanen et al.,
2012) and Analogue (Simpson and Oksanen, 2012). Further
enhancement of the sea-ice reconstruction was obtained by consideration of
the abundance pattern of the diatom sea-ice indicators, allowing for
qualitative estimation of sea-ice occurrence, as proposed by Gersonde and
Zielinski (2000).
Sea ice data
The mean monthly satellite sea ice concentration was derived from Nimbus-7
SMMR and DMSP SSM/I-SSMIS passive microwave data and downloaded from the
National Snow and Ice Data Center (NSIDC; Cavalieri et al.,
1996). The sea ice concentration is expressed to range from 0 % to 100 %,
with concentrations below 15 % suggesting the minor occurrence of sea
ice. Accordingly, the sea ice extent is defined as the ocean area with a sea
ice cover of at least 15 %.
An interval from 1980 to 2015 was used to generate an average sea ice
distribution for each season: spring (SON), summer (DJF), autumn (MAM) and
winter (JJA) (Table 2), and the data are considered to reflect the modern mean
state of sea ice coverage around the WAP. The high standard deviation in the
seasonal sea ice concentrations (up to 26 % in winter; Table 2) in the
vicinity of the WAP is attributed to the distinct intra- and interannual
variability in sea ice coverage. In this regard, Kim et al. (2005) already related
interannual changes in particle flux to annual changes in sea ice cover in
the Bransfield Strait. We here suggest that considering mean sea ice
concentrations determined for an observational period of 35 years reflects
a good estimate of average sea ice conditions and facilitates comparison
with sedimentary archives.
Results and discussion
In the following we present and discuss the sedimentary concentrations of
IPSO25, HBI trienes and phytosterols regarding their spatial
distribution patterns in relation to the environmental conditions and
oceanographic features in the study area. We especially focus on the
applicability of these biomarkers for reconstructing sea ice conditions and
integrate information derived from satellite observations and diatom-based
sea ice estimations. We further discuss the possible approach of a sea ice
index PIPSO25 by analogy with the Arctic sea ice index PIP25
(Müller et al., 2011).
Biomarker distributions in surface sedimentsDistribution of IPSO<inline-formula><mml:math id="M178" display="inline"><mml:msub><mml:mi/><mml:mn mathvariant="normal">25</mml:mn></mml:msub></mml:math></inline-formula>
The sea ice biomarker IPSO25 was detected in 14 samples, with
concentrations ranging between 0.37 and 17.81 µg g-1 TOC (Table 1). The distribution of IPSO25 in the study area shows a clear
northwest–southeast gradient (Fig. 3a) with concentrations increasing from
the continental slope and around area from the South Shetland Islands to the
continental shelf. Maximum IPSO25 concentrations are observed at
stations under TWW influence with distinctly cold summer SSTs in the
Bransfield Strait. According to Belt et al. (2016), deposition of
IPSO25 is highest in areas covered by landfast sea ice and platelet ice
during early spring and summer. Platelet ice is formed under supercooling
ocean conditions in the vicinity of ice shelves and subsequently may be
incorporated into drifting sea ice (Gough et al.,
2012; Hoppmann et al., 2015). We note that, for example, core sites
PS97/068, PS97/069, PS97/072 and PS97/073 in the central and eastern
Bransfield Strait are located too distal to be covered by fast ice and
suggest that peak IPSO25 concentrations at these sites may refer to the
frequent drift and melt of sea ice exported from the Weddell Sea into the
Bransfield Strait. The vertical export of biogenic material from sea ice
towards the seafloor may be accelerated significantly by the formation of
organic-mineral aggregates, fecal pellets or by (cryogenic) gypsum
ballasting, which promotes a rapid burial and sedimentation of organic
matter in polar settings (De
La Rocha and Passow, 2007; Wefer et al., 1988; Wollenburg et al., 2018). A
recent study from Schmidt et al. (2018) shows that the
occurrence of IPSO25 in suspended matter and pelagic grazers (krill) is
closely linked to the position of the sea ice edge. Lateral subsurface
advection of organic matter (including biomarkers) through the TWW, however,
may also contribute to elevated IPSO25 concentrations at these sites.
IPSO25 was not detected in sediments from the permanently ice-free
areas in the Drake Passage.
Distribution of (a) IPSO25, (b) HBI Z-triene, (c) HBI E-triene,
(d) brassicasterol and (e) dinosterol concentrations normalized to TOC. All
distribution plots were made with Ocean Data View (2017).
The δ13C values of IPSO25 are between -10.3 ‰ and -14.7 ‰, which is the commonly
observed range for IPSO25 in surface sediments, sea-ice-derived organic
matter and in Antarctic krill stomachs (Belt
et al., 2016; Massé et al., 2011; Schmidt et al., 2018). These values
contrast the low δ13C values of marine phytoplankton lipids in
Antarctic sediments (-38 ‰ to -41 ‰ after Massé et al., 2011) and support the sea ice
origin of IPSO25 in the study area.
Distribution of HBI trienes
The HBI Z-triene was present in all 26 samples (0.33–26.86 µg g-1 TOC) and the HBI E-triene was found in 24 samples (0.15–13.87 µg g-1 TOC) (Table 1). The highest concentrations of both HBI
trienes are found in the eastern Drake Passage and along the continental
slope, where IPSO25 is absent, while their concentrations in the
Bransfield Strait are generally low (Fig. 3b and c), suggesting unfavourable
environmental conditions for their source diatoms (e.g. cooler SSTs, sea
ice cover, grazing pressure). Contrary to the finding of elevated HBI
Z-triene concentrations in surface waters along an ice edge
(Smik et al., 2016a) and
earlier suggestions that this biomarker may be used as a proxy for MIZ
conditions (Belt
et al., 2015; Collins et al., 2013; Schmidt et al., 2018), we observe
the highest concentrations of the HBI Z- and E-triene at the permanently
ice-free northernmost stations PS97/083 and PS97/084 in the eastern Drake
Passage. These core sites are located close to the Antarctic
Polar Front (Fig. 2), and we assume that the productivity of HBI triene
source diatoms may benefit from mixing and upwelling of warm and cold water
masses in this area (Moore and Abbott,
2002). Sediments collected south of the Antarctic Polar Front and along the
Hero Fracture Zone in the western Drake Passage (Fig. 2) contain moderate
and very low concentrations of HBI trienes, respectively. The Hero Fracture
Zone is mainly barren of fine-grained sediments and dominated by sands
(Lamy, 2016), which may point to intensive winnowing by ocean
currents impacting the deposition and burial of organic matter. Moderate
concentrations of HBI trienes at the continental slope along the WAP
(PS97/053, PS97/074, PS97/077) and in the Bransfield Strait likely refer to
primary production associated with the retreating sea ice margin during
spring and summer. This indicates seasonally ice-free waters in high
production coastal areas influenced by upwelling (Gonçalves-Araujo et al.,
2015) and feeding of the local food web (Schmidt et al., 2018). The
similarity in the distribution of the HBI Z- and the E-triene in our surface
sediments – the latter of which is so far not often considered for Southern
Ocean paleoenvironmental studies – supports the assumption of a common
diatom source for these HBIs (Belt
et al., 2000, 2017).
We consider that degradation of biomarker lipids may affect their
distribution within surface sediments. While laboratory studies on HBIs in
solution point to a low reactivity of IPSO25 towards auto- and
photooxidative degradation (Rontani
et al., 2014, 2011), a more recent investigation into Antarctic surface
sediments shows that IPSO25 may potentially be affected by partial
autoxidative and bacterial degradation, but oxidation products are found in
only minor proportions (Rontani et al.,
2019a). Since HBI trienes exhibit a generally higher sensitivity to
degradation than the C25 HBI diene (Rontani
et al., 2014, 2019b) – and this is supported by a recent observation of
increasing IPSO25/HBI triene ratios with increasing water depths in a
polynya system off Eastern Antarctica (Rontani et al., 2019b) –
their lower concentrations in the Bransfield Strait have to be considered
with care. Vice versa, regarding maximum HBI triene concentrations and the
absence of IPSO25 in Drake Passage sediments, we conclude that the
absence of the latter in these samples can be linked to the lack of sea ice
(and not to the degradation of IPSO25 as HBI trienes would have been
removed first).
Distribution of sterols
Brassicasterol is present in all samples, with concentrations ranging from
3.39 to 5017.44 µg g-1 TOC, while dinosterol was detected in 22
samples (0.0002–1983.75 µg g-1 TOC). It is noticeable that the
concentrations of sterols exceed the concentrations of IPSO25 and HBI
trienes by more than 2 orders of magnitude. We observe higher
concentrations of brassicasterol and dinosterol in the eastern part of the
Drake Passage, supporting an open marine source for these sterols.
Surprisingly, elevated concentrations of brassicasterol are also found at
stations PS97/048-1 and 049-2 in the Hero Fracture Zone, which may argue
against a winnowing signal, leading to lower accumulation of organic matter.
We can only speculate if transport and deposition of reworked sediment
containing brassicasterol via iceberg rafting could explain these higher
values. In contrast to the observation made for HBI trienes, high sterol
concentrations are found in the eastern and central Bransfield Strait (Fig. 3d and e). Previously, elevated concentrations of steroidal components
including brassicasterol and dinosterol in sediment cores from the
Bransfield Strait have been interpreted to reflect a high productivity and
significant inputs from diatoms and dinoflagellates
(Brault and Simoneit, 1988). In a more recent overview,
Cárdenas et al. (2018) also report peak
concentrations of pigments, sterols and total organic carbon in the
Bransfield Strait, which they relate to large seasonal phytoplankton blooms
and higher accumulation rates. Dinosterol and, in particular, brassicasterol
are known to have different source organisms including diatoms,
dinoflagellates, cryptophytes, prymnesiophycean algae and cyanobacteria
(Volkman, 1986), and we assume that this diversity accounts for
the higher concentration of these lipids in Bransfield Strait sediments,
while concentrations of HBI trienes, mainly derived from diatoms, are
significantly lower. Regarding the potential input of brassicasterol from
cryptophytes (Gladu et al.,
1990; Goad et al., 1983), changes in the dominance of this phytoplankton
group over diatoms have been reported for our study area and have been
associated with a shallowing of the mixed layer and lower salinity due to
intensified glacial ice-melting along the WAP
(Mendes et al., 2013).
Similar to the observations made for HBIs, selective degradation may also
affect the concentration of phytosterols within surface sediments. With
respect to the preservation potential of terrigenous and marine derived
sterols, Rontani et al. (2012)
note only a weak effect of biotic and abiotic degradation of brassicasterol
in Arctic Ocean shelf sediments – whether this is also true for Southern Ocean
shelf areas needs to be determined. In general, further investigations into
degradation processes affecting both HBIs and phytosterols within (the same)
sediment samples would address an important knowledge gap regarding in situ
biochemical modifications of the biomarker signal.
Comparison of satellite-derived modern sea ice conditions and biomarker data
The spring and winter sea ice concentrations are shown in Fig. 4a and b.
Winter sea ice is estimated to not extend north of 61∘ S (Fig. 4b)
and varies between 1 % and 50 % in the study area, while sea ice is
reduced to less than 20 % in spring (Fig. 4a, Table 2). Sea ice
concentrations of up to 50 % are common in winter between the South
Shetland Islands and north of the Antarctic Sound, where the influence of TWW
is highest. Permanent sea ice cover is uncommon in the Bransfield Strait and
around the WAP and this area is mainly characterized by a high sea ice
seasonality, drift ice from the Weddell Sea
(Collares et al., 2018) and a seasonally
fluctuating sea ice margin.
Seasonal sea ice concentrations from satellite observations for
spring, summer, autumn and winter, with standard deviations.
The satellite-derived mean sea ice concentrations at each sampling
station for (a) spring and (b) winter.
Comparisons of IPSO25 and winter sea ice concentrations derived from
satellite data reveal a positive correlation (r2=0.53). The
strongest relationship is observed in the eastern Bransfield Strait where
the influence of TWW is high. Correlations with spring sea ice (r2=0.27) and other seasons are weak. As photosynthesis is not possible and a
release of sea ice diatoms from melting sea ice is highly reduced during the
Antarctic winter, the observation of a stronger correlation between recent
winter sea ice concentrations and IPSO25 is unexpected. We hence
suggest that this offset may be related to the fact that the sediment
samples integrate a longer time interval than is covered by satellite
observations. Radiocarbon dating of selected samples that contain calcareous
material reveals an age of 100 years BP in the vicinity of the South
Shetland Islands (station PS97/059-2) and 142 years BP at the Antarctic
Sound (station PS1546-2, Table 3). A significantly older age was determined
for a sample of N. pachyderma from station PS97/044-1 (4830 years BP) which likely
denotes the winnowing and/or very low sedimentation rates in the Drake
Passage. Bioturbation effects and uncertainties in reservoir ages
potentially mask the ages of the near-coastal samples. Nevertheless, since
other published ages of surface sediments within the Bransfield Strait (Barbara
et al., 2013; Barnard et al., 2014; Etourneau et al., 2013; Heroy et al.,
2008) are also in the range of 0–270 years, we consider that our surface samples
likely reflect the paleoenvironmental conditions that prevailed during the
last two centuries (and not just the last 35 years covered by satellite
observations). In the context of the rapid warming during the last century
(Vaughan et al., 2003) and the decrease in sea ice at
the WAP (King, 2014; King and Harangozo, 1998), we
suggest that the biomarker data of the surface sediments relate to spring
sea ice cover, which must have been enhanced compared to the recent (past 35 years) spring sea ice recorded via remote sensing. Presumably, the average
spring sea ice conditions over the past 200 years might have been similar to
the modern (past 35 years) winter conditions, which would explain the
stronger correlation between IPSO25 and winter sea ice concentrations.
The absence of IPSO25 at stations PS97/052 and PS97/053, off the
continental slope, is in conflict with the satellite data depicting an
average winter sea ice cover of 23 %. Earlier documentations that the
IPSO25-producing sea ice diatom Berkeleya adeliensis favours landfast ice communities in
East Antarctica and platelet ice occurring mainly in near-coastal areas (Belt
et al., 2016; Riaux-Gobin and Poulin, 2004) could explain this mismatch
between biomarker and satellite data, which further strengthens the
hypothesis that the application of IPSO25 seems to be confined to
continental shelf or near-coastal and meltwater-affected environments (Belt,
2018; Belt et al., 2016). Alternatively, strong ocean currents (i.e. the
ACC) could have impacted the deposition of IPSO25 in this region.
Details of the radiocarbon dates and calibrated ages.
Although the distribution pattern of HBI trienes reveals generally higher
concentrations in ice-free environments, we note only very weak negative
correlations with satellite sea ice data (r2<0.1). This may
relate to the strong spatial variability in HBI triene concentrations within
the Drake Passage and the different time periods represented by the
satellite and sediment data. Similar to the HBI trienes, the sterols also do
not show any significant relationship to the satellite sea ice
concentrations. High abundances of brassicasterol and dinosterol are
observed in both ice-free as well as in seasonally ice-covered regions,
which points to a broad environmental adaptation of the source organisms. We
hence consider that other environmental parameters than sea ice (e.g.
nutrient availability, water temperature and/or grazing pressure) exert
major control on the productivity of HBI triene and sterol producers in the
study area.
Comparison of biomarker distributions and diatom-based sea ice estimates
The diatoms preserved in sediments from the study area (Table 4) can be
associated with open-ocean and sea ice conditions (Fig. 5a–d). North of the
South Shetland Islands, the strong influence of the ACC is reflected in the
high abundance of open-ocean diatom species such as Fragilariopsis kerguelensis and Thalassiosira lentiginosa (Esper et al.,
2010). The two diatom species Fragilariopsis curta and Fragilariopsis cylindrus – known to not produce HBIs (Belt
et al., 2016; Sinninghe Damsté et al., 2004) – mark the vicinity to sea
ice (Buffen
et al., 2007; Pike et al., 2008) and indicate fast and melting ice, a stable
sea ice margin and stratification due to melting processes and the
occurrence of seasonal sea ice. These observations are in accordance with
previous diatom studies, revealing a dominance of Fragilariopsis kerguelensis in the permanently
open-ocean zone in the Drake Passage and an assemblage shift to more cold-water-adapted and sea-ice-associated species in the seasonal sea ice zone of
the Bransfield Strait (Cárdenas et al., 2018).
Estimations of winter sea ice (WSI) derived from diatom species and
the distribution of main diatom species in each sample.
Distribution of the diatoms (a)T. lentiginosa, (b)F. kerguelensis, (c)F. curta and (d)F. cylindrus in the study
area (percentage per sample). The winter sea ice concentrations from the
application of transfer function of Esper and Gersonde (2014a)
are shown in (e).
The high abundance of these sea ice diatoms in our samples is in good
agreement with high and moderate IPSO25 concentrations in the
Bransfield Strait and around the South Shetland Islands, respectively. The
only HBI source diatom identified is the HBI Z-triene-producing
Rhizosolenia hebetata (Belt et al., 2017),
which is present in four samples in relatively small amounts and does not show a relation to the measured HBI Z-triene concentrations (Tables 1 and 4).
The source diatom of IPSO25Berkeleya adeliensis was not observed (or preserved) in the
samples, and we suggest that additional, hitherto unknown, producers for
IPSO25 as well as for the HBI trienes may exist.
We applied the transfer function of Esper and Gersonde (2014a)
with four analogues (4an, Table 4) to our samples to estimate winter sea ice
concentrations (Fig. 5e). The diatom approach shows a clear trend of
high winter sea ice concentrations in the range of 78 %–91 % in the
Bransfield Strait and low sea ice concentrations (6 %–39 %) north
of the continental slope. The fact that diatom data propose sea ice in the
Drake Passage may result from the high ages of surface sediments but also
from drift, resuspension and sedimentation of diatom remains. Because of the
absence of IPSO25 in the Drake Passage the correlation of its
concentrations with WSI is only weak (r2=0.29).
Testing a semi-quantitative sea ice approach for the Southern Ocean: PIPSO<inline-formula><mml:math id="M237" display="inline"><mml:msub><mml:mi/><mml:mn mathvariant="normal">25</mml:mn></mml:msub></mml:math></inline-formula>
Following the PIP25 approach applied in the Arctic Ocean (Müller
et al., 2011; Belt and Müller, 2013; Xiao et al., 2015), we used
IPSO25, HBI triene and sterol data to calculate the PIPSO25 index.
The main concept of combining the sea ice proxy with an indicator of an
ice-free ocean environment (i.e. a
phytoplankton biomarker; Müller et al., 2011) aims for a more detailed
assessment of the sea ice conditions. By reducing the light penetration
through the ice, a thick and perennial sea ice cover limits the productivity
of bottom sea ice algae (Hancke et al.,
2018), which results in the absence of both sea ice and pelagic
phytoplankton biomarker lipids in the underlying sediments. Vice versa,
sediments from permanently ice-free ocean areas only lack the sea ice
biomarker but contain variable concentrations of phytoplankton biomarkers
(Müller et al., 2011). The
co-occurrence of both biomarkers in a sediment sample suggests seasonal sea
ice coverage promoting algal production indicative of sea ice as well as
open-ocean environments (Müller et
al., 2011). Consideration of a phytoplankton biomarker alongside the sea ice
proxy hence helps to avoid an underestimation of the past sea ice cover
deduced from the absence of the sea ice proxy, which, in fact, may also be
due to permanent sea ice cover (Belt, 2018,
2019; Belt and Müller, 2013).
Depending on the biomarker reflecting pelagic (open-ocean) conditions, we
here define PZIPSO25 (using the HBI Z-triene), PEIPSO25
(using the HBI E-triene), PBIPSO25 (using brassicasterol) and
PDIPSO25 (using dinosterol).
The PIPSO25 values are 0 in the Drake Passage and increase to
intermediate values at the South Shetland Islands and the continental slope
and reach highest values in the Bransfield Strait (Fig. 6a–d). Minimum
PIPSO25 values are supposed to refer to a predominantly ice-free
oceanic environment in the Drake Passage, while moderate PIPSO25 values
mark the transition towards a marginal sea ice coverage at the continental
slope and around the South Shetland Islands. Elevated PIPSO25 values in
samples from the northeastern Bransfield Strait suggest an increased sea ice
cover (probably sustained through the drift of sea ice originating in the
Weddell Sea). This pattern reflects the oceanographic conditions of a
permanently ice-free ocean north of the South Shetland Islands and a
seasonal sea ice zone at the WAP influenced by the Weddell Sea as described
by Cárdenas et al. (2018). Both HBI triene-based
PIPSO25 indices show constantly high values at the coast of the WAP of
>0.7 (PZIPSO25) and >0.8
(PEIPSO25), respectively, and in the southern Bransfield Strait
paralleling the southwest–northeast oriented Peninsula Front described by
Sangrà et al. (2011). This front
is reported to act as a barrier for phytoplankton communities
(Gonçalves-Araujo et al.,
2015) and is associated with the encounter between TWW carrying Weddell Sea
sea ice through the Antarctic Sound and the TBW. The high PIPSO25
values suggesting extended sea ice cover west of the Peninsula Front
(station PS97/054 and PS97/056) result from minimum concentrations of
pelagic biomarkers and moderate concentrations of IPSO25. PIPSO25
values based on the HBI E-triene are about 0.2 higher compared to
PZIPSO25, due to the generally lower concentrations of the HBI
E-triene (Table 1).
Distribution of (a) PZIPSO25, (b) PEIPSO25, (c) PBIPSO25 and (d) PDIPSO25 values in the study area. The
extent of 15 %, 30 %, 40 % and 50 % satellite sea ice
concentrations during winter is added as contour lines (see also Fig. 4b).
The sterol-based PIPSO25 values display a generally similar pattern to those of PZIPSO25 and PEIPSO25, respectively, and we note a
high comparability between the PEIPSO25 and PBIPSO25 values
(r2=0.73). Some differences, however, are observed in the
southwestern part of the Bransfield Strait (station PS97/056), where
PBIPSO25 indicates a lower sea ice cover, and in the central
Bransfield Strait (stations PS97/068 and PS97/069), where PBIPSO25
and PDIPSO25 point to only MIZ conditions. Regarding the modern
sea ice conditions, the HBI triene-based PIPSO25 indices hence seem to
reflect the oceanographic conditions within the Bransfield Strait more
satisfactorily. It should be noted that the brassicasterol- or
dinosterol-based PIPSO25 index links environmental information derived
from biomarker lipids belonging to different compound classes (i.e. HBIs and
sterols), which have fundamentally different chemical properties. This
requires special attention as, for example, selective degradation of one of
the compounds may affect the sedimentary concentration of the respective
lipids (Rontani et al., 2018). Previous
studies linking HBI and sterol-based sea ice reconstructions with
satellite-derived or, with respect to downcore paleo-studies, paleoclimatic
data, however, demonstrate that the climatic and/or environmental conditions
controlling the production of HBIs and sterols seem to exceed the influence
of a potential preferential degradation of these biomarkers within the
sediments (e.g.
Berben et al., 2014; Cabedo-Sanz et al., 2013; Müller et al., 2009,
2012; Müller and Stein, 2014; Stein et al., 2017; Xiao et al., 2015). A
comparison of PIP25 records determined using brassicasterol and the HBI
Z-triene for three sediment cores from the Arctic realm covering the past up
to 14 000 years BP (Belt et
al., 2015) reveals very similar trends for both versions of the PIP25
index in each core, which may point to, at least, a similar degree of
degradation of HBI trienes and sterols through time. More such studies are
needed to evaluate the preservation potential of HBIs and sterols in
Southern Ocean sediments, especially for down core paleo-studies.
Since brassicasterol and dinosterol are highly abundant in both seasonally
ice-covered Bransfield Strait sediments as well as in permanently ice-free
Drake Passage sediments, their use as an indicator of fully open-ocean
conditions in the study area is questionable. Elevated concentrations of
both sterols in the Bransfield Strait could either point to an additional
input of these lipids from melting sea ice (Belt et al., 2013) or a better
adaptation of some of their source organisms to cooler and/or ice-affected
ocean environments. Production and accumulation of these lipids in (late)
summer (i.e. after the sea ice season) has to be considered as well. This
observation highlights the need for a better understanding of the source
organisms and the mechanisms involved in the synthesis of these sterols.
Similarly, more research is needed on the production of IPSO25 in
Southern Ocean sea ice environments. The source diatom Berkeleya adeliensis seems to be
restricted to a very unique ice environment. Previous studies documenting
the lack of IPSO25 in distal, though winter-sea-ice-covered, areas
(e.g. Belt et al.,
2016) emphasize this limitation, and it has been suggested that IPSO25
may be more indicative of the type of sea ice rather than sea ice extent
(Belt, 2019), which needs to be considered when
targeting at more quantitative sea ice reconstructions using this biomarker.
Scatter plots of satellite spring sea ice concentrations and (a) PZIPSO25 (triangles, solid regression line) and PEIPSO25
(crosses, dashed regression line) and (b) PBIPSO25 (triangles,
solid regression line) and PDIPSO25 (circles, dashed regression
line). Scatter plots of satellite winter sea ice concentrations with (c) PZIPSO25 (triangles, solid regression line) and PEIPSO25
(crosses, dashed regression line) and (d) PBIPSO25 (black
triangles, solid regression line) and PDIPSO25 (circles, dashed
regression line). All scatter plots were done with Grapher™ 13.
Scatter plots of (a) PZIPSO25 (triangles, solid
regression line) and PEIPSO25 (crosses, dashed regression line)
and (b) PBIPSO25 (triangles, solid regression line) and
PDIPSO25 (circles, dashed regression line) against diatom-derived
winter sea ice concentrations. (c) Scatter plot of diatom transfer function
winter sea ice concentrations and satellite winter sea ice concentrations.
Comparison of PIPSO<inline-formula><mml:math id="M319" display="inline"><mml:msub><mml:mi/><mml:mn mathvariant="normal">25</mml:mn></mml:msub></mml:math></inline-formula> with satellite sea ice data and diatom sea ice estimations
In the northeastern part of the study area, the HBI-triene-based PIPSO25 indices align well with winter sea ice concentrations and
depict the gradient from the marginally ice-covered southern Drake Passage
towards the intensively ice-covered Weddell Sea. This is visualized with
contour lines from the observed sea ice extent of 15 %, 30 %, 40 %
and 50 % winter sea ice compared to the PIPSO25 values in Fig. 6a–d. In the southwestern part of the Bransfield Strait, all PIPSO25
indices suggest higher sea ice cover than is reflected in the satellite
data. This may be explained by the transport (and melt) of drift ice through
the TWW, joining the TBW at the southwestern Peninsula Front and/or a higher
sea ice cover in this area prior to the remote sensing observational period
(and prior to the recent WAP warming).
Correlations of PIPSO25 values with satellite-derived sea ice
concentrations (for spring, summer, autumn and winter) contrast with earlier
observations made for the PIP25 index in the Arctic Ocean, where the
closest linear relationship is found mainly with the spring sea ice coverage
(i.e. the
blooming season of sea ice algae; Müller et al., 2011; Xiao et al.,
2015). We observe a remarkably low correlation between PIPSO25 values
and spring sea ice concentrations of less than 20 % with a coefficient of
determination r2=0.37 for PZIPSO25, r2=0.50 for
PEIPSO25 (Fig. 7a), r2=0.31 for PBIPSO25 and
r2=0.34 for PDIPSO25 (Fig. 7b). The highest correlation
is observed between winter sea ice concentrations and PEIPSO25
(r2=0.72), and between winter sea ice concentrations and PZIPSO25 (r2=0.65, Fig. 7c), with
a weaker correlation for the sterol-based PIPSO25 values
(PBIPSO25: r2=0.52; PDIPSO25: r2=0.44,
Fig. 7d). As discussed above, we attribute this seemingly conflicting result
of a better agreement between biomarker data and winter (instead of spring)
sea ice conditions to the offset in the time intervals reflected in
satellite and sediment data. For the application of the PIPSO25
approach, more aspects concerning the physical environmental conditions
controlling the formation of platelet ice, which, at least at this state of
research, is regarded as a major source of IPSO25
(Belt et al., 2016) need to
be considered. The formation and accumulation of platelet ice in supercooled
waters below landfast sea ice or underneath an ice shelf (e.g.
Gough et al., 2012; Hoppmann et al., 2015) seem to limit the spatial
occurrence of IPSO25 and hence the applicability of PIPSO25 to
coastal environments. However, transport of supercooled waters away from the
coast may lead to platelet ice formation (and colonization of Berkeleya adeliensis) in more
distal areas (Hoppmann et al., 2015), and also the
drift of sea ice (including the underlying platelet ice) may impact the
distribution of IPSO25 in Southern Ocean sediments; these processes
require further investigations. Even though PIPSO25 values show a
stronger relationship to satellite sea ice concentrations than IPSO25
concentrations, more insight into the production and sedimentation of the
biomarker lipids involved is needed to develop such a semi-quantitative
approach.
With regard to the spatially and temporally variable sea ice extent, Esper
and Gersonde (2014a)
studied the response of diatom species to changes in environmental
conditions and their response to the non-linear behaviour of sea ice dynamics (Zwally
et al., 2002). In contrast to ice-free areas or areas of permanent sea ice
cover, areas characterized by the transition from consolidated to
unconsolidated sea ice show rapid changes in satellite-derived sea ice
concentrations (ranging from 90 % to 15 %) and exhibit a large
variability in species composition. To reflect this curve in sea ice we
hence chose a cubic polynomial regression (polynomial of the third degree) to
determine the relation between PIPSO25 values and satellite data
depicting sea ice concentrations of more than 20 %. A slightly
sigmoid-shaped regression line of winter sea ice concentrations and
PIPSO25 values depicts the non-linearity of sea ice cover in different
sea ice regimes.
A positive correlation is found between WSI concentrations derived from
diatoms and the PIPSO25 indices based on HBI trienes
(PZIPSO25 with r2=0.76; PEIPSO25 with r2=0.77, Fig. 8a). The correlations of sterol-based PIPSO25 values
with WSI are slightly lower but in the same range (PBIPSO25 with
r2=0.74; PDIPSO25 with r2=0.69, Fig. 8b). A
slightly weaker correlation is noted for diatom- and satellite-based winter
sea ice concentrations (r2=0.63; Fig. 8c). Overall, the diatom
approach indicates higher sea ice concentrations than the satellite data,
with an offset of up to 65 %. This may be due to different sources of
satellite reference data used for the transfer function or also due to the
fact that the sediment samples integrate for a longer time period with a higher
sea ice cover than the satellite data (see discussion in Sect. 4.2).
Regarding future sea ice reconstructions based on IPSO25 and other
biomarkers, we note that the simultaneous study of diatom assemblages
provides valuable information on the sea surface conditions and may help to
avoid misleading interpretation of the biomarker data
(Belt, 2019). Vice versa, while diatom-based transfer
functions mainly refer to winter sea ice concentrations, the IPSO25
(and PIPSO25) signal holds critical information on coastal
spring and/or summer sea ice conditions, which are often crucial for ice-shelf
(melting) processes. Pairing the micropaleontological and the biomarker
approach hence provides for a more comprehensive reconstruction of Southern
Ocean sea ice conditions.
Conclusions
The distribution of the sea ice biomarker IPSO25, related HBI trienes
and phytosterols, and diatoms in a suite of surface sediments from the
southern Drake Passage and the WAP reflects recent sea surface water
characteristics reasonably well. While highest HBI triene concentrations are
observed in the permanently open-ocean zone of the Drake Passage, they are
significantly reduced in the seasonally ice-covered Bransfield Strait. This
pattern is reversed for the sea ice proxy IPSO25 and in accordance with
previous surface sediment analyses, revealing a preferential occurrence of
this biomarker in near-coastal environments. The distribution of
phytosterols points to a broader environmental significance of
brassicasterol and dinosterol in terms of ocean temperature and sea ice
tolerance, and/or nutrient availability. Following the PIP25 approach
established for Arctic Ocean sea ice reconstructions, the herein proposed
sea ice index PIPSO25 indicates seasonal sea ice cover along the coast
of the WAP and in the Bransfield Strait, whereas mainly ice-free conditions
prevail in the Drake Passage. In general, this pattern is consistent with
satellite-derived sea ice data and diatom-based sea ice estimates, and we
note that the PIPSO25 index seems to be a potential approach towards
semi-quantitative sea ice reconstructions in the Southern Ocean. The recent
rapid warming in the study area, however, affects the comparability of proxy
and satellite data. The fact that the surface sediments integrate a
significantly longer time interval than the remote sensing data thwarts
attempts to calibrate PIPSO25 values against observed sea ice
concentrations. Additional data from other circum-Antarctic coastal (and
distal) environments and investigations into potential calibration methods
are needed to further develop this approach. Importantly, more information
is needed on the mechanisms of IPSO25 and HBI triene synthesis,
transport and preservation within sediments. Despite a generally good
agreement between PIPSO25-, diatom- and satellite-based sea ice
distributions, we note that the basically different sea ice patterns and sea
ice varieties in the Southern Ocean and accordingly different mechanisms
controlling the IPSO25 signal need to be considered carefully, when
adapting a (not yet fully validated) semi-quantitative approach initially
developed for the Arctic Ocean.
Data availability
All data can be found in this paper and are available at 10.1594/PANGAEA.897165 (Vorrath et al., 2019).
The supplement related to this article is available online at: https://doi.org/10.5194/bg-16-2961-2019-supplement.
Author contributions
The study was conceived by MEV and JM. Data collections and experimental
investigations were done by MEV together with OE (diatoms), GM (radiocarbon
dating), CH (satellite data) and ES (isotope data). MEV wrote the paper
and did the visualizations. KF provided technical support. JM supervised the
study. All authors contributed to the interpretation and discussion of the
results and the conclusion of this study.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
We thank the captain, crew and chief scientist Frank Lamy of RV Polarstern
cruise PS97, and the following supporters: Mandy Kiel and Denise Diekstall
(technicians), Lester Lembke-Jene (biology, dating), Liz Bonk and Hendrik Grotheer (from MICADAS), Max Mues (sample preparation), Nicoletta Ruggieri
(lab support), and Walter Luttmer (lab support). Simon Belt is acknowledged for
providing the 7-HND internal standard for HBI quantification. We also
acknowledge the two anonymous reviewers and the editor for their
constructive and detailed comments. Financial support was provided through
the Helmholtz Research grant VH-NG-1101.
Financial support
The article processing charges for this open-access publication were covered by a Research Centre of the Helmholtz Association.
Review statement
This paper was edited by Marcel van der Meer and reviewed by two anonymous referees.
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