The North Water Polynya (NOW, Inuktitut: Sarvarjuaq; Kalaallisut: Pikialasorsuaq), Baffin Bay, is the largest polynya and one of the most productive regions in the Arctic. This area of thin to absent sea ice is a critical moisture source for local ice sheet sustenance and, coupled with the inflow of nutrient-rich Arctic Surface Water, supports a diverse community of Arctic fauna and indigenous people. Although paleoceanographic records provide important insight into the NOW's past behavior, it is critical that we better understand the modern functionality of paleoceanographic proxies. In this study, we analyzed lipid biomarkers, including algal highly branched isoprenoids and sterols for sea ice extent and pelagic productivity and archaeal glycerol dibiphytanyl glycerol tetraethers (GDGTs) for ocean temperature, in a set of modern surface sediment samples from within and around the NOW. In conjunction with previously published datasets, our results show that all highly branched isoprenoids exhibit strong correlations with each other and not with sterols, which suggests a spring or autumn sea ice diatom source for all highly branched isoprenoids (HBIs) rather than a combination of sea ice and open-water diatoms as seen elsewhere in the Arctic. Sterols are also highly concentrated in the NOW and exhibit statistically higher concentrations here compared to sites south of the NOW, consistent with the order of magnitude higher primary productivity observed within the NOW relative to surrounding waters in spring and summer months. Finally, our local temperature calibrations for GDGTs and OH-GDGTs reduce the uncertainty present in global temperature calibrations but also identify some additional variables that may be important in controlling their local distribution, such as nitrate availability and dissolved oxygen. Collectively, our analyses provide new insight into the utility of these lipid biomarker proxies in high-latitude settings and will help provide a refined perspective on the past development of the NOW with their application in downcore reconstructions.
Arctic and Antarctic polynyas are key sites for deep water formation
(Kuhlbrodt et al., 2017), ice sheet moisture (Smith et al., 2010), and
enhanced productivity that can sequester atmospheric CO
Here, we focus on the North Water Polynya (NOW, Inuktitut: Sarvarjuaq; Kalaallisut:
Pikialasorsuaq), which is the largest polynya (85 000 km
Overview map of Baffin Bay. Simplified ocean surface currents
are shown using bold lines, where red reflects warm Atlantic Water (West Greenland
Current, WGC) and blue reflects cool Arctic Water (Baffin Current, BC). To
the north is the June limit of the NOW (dotted purple line). Seasonal sea
ice limits are shown with dotted (autumn), dashed (spring), and solid black lines
(winter) (Cavalieri et al., 1996). The numbered locations of this study's
modern sites are shown with red circles (
In this study, we aim to characterize the NOW through the distribution of lipid biomarkers archived in marine seafloor surface sediments that encompass its modern area in Baffin Bay using both new and previously published data (Kolling et al., 2020). We focus on different lipid classes that inform us about seasonal sea ice extent, surface productivity, and ocean temperature. Our assessment of these biomarker proxies against modern instrumental data (i.e., satellite-derived sea ice extent and in situ environmental datasets) provides a key baseline for interpreting the presence and extent of the NOW in the geologic past (Georgiadis et al., 2020; Jackson et al., 2021). On a broader scale, our work is also critical for the community's general understanding of these lipids' environmental relationships at high northern latitudes where some proxy datasets are currently sparse (e.g., Tierney and Tingley, 2014, 2018). For example, existing biomarker temperature calibrations are often global in scale and feature high uncertainties at the low end of the temperature spectrum (Kim et al., 2010, 2012). However, uncertainty can be substantially reduced by filling in high-latitude regions and isolating their distinct characteristics with local calibrations (Tierney and Tingley, 2014, 2018; Harning et al., 2019). While some of our biomarker datasets are limited in size, they provide an important first step for the continued refinement of these proxies at high latitudes.
Ocean circulation in Baffin Bay is cyclonic, involving the northward-flowing West Greenland Current (WGC) and the southward-flowing Baffin Current (BC) (Fig. 1). The warm and saline WGC carries a mixture of Atlantic Water from the Irminger Current and Polar Water from the East Greenland Current, whereas the BC is comprised of low-salinity Arctic Surface Water (ASW) that enters Baffin Bay from the Arctic Ocean through the Canadian Arctic Archipelago (CAA) channels. The ASW is modified by mixing with terrestrial-derived freshwater and by sea ice processes en route to Baffin Bay (Tang et al., 2004; Münchow et al., 2006, 2015; Azetsu-Scott et al., 2010). The present-day depths of the CAA channels govern the composition of inflowing ASW (Jones et al., 2003); Nares Strait has a sill depth of 220 m that allows passage of both the Polar Mixed Layer (containing high-nutrient Pacific Water from Bering Strait) and some of the halocline layer that has been mixed with the underlying Atlantic layer of the Arctic Ocean (Azetsu-Scott et al., 2010). Lancaster and Jones Sounds have shallower sill depths that exclude all but the most carbonate-undersaturated Polar Mixed Layer (Azetsu-Scott et al., 2010). These Arctic outflows join the BC and form the upper 100 to 300 m of surface water in Baffin Bay, except where the WGC dominates in the southeast (Tang et al., 2004).
Sea ice covers nearly all of Baffin Bay in winter, except in the southeast due to the warmth and salinity of the WGC (Fig. 1). Sea ice begins to form in September and reaches maximum coverage in March and is thickest along the Baffin Island coast where the ASW flow is concentrated (Fig. 1, Tang et al., 2004). In contrast, the NOW has low concentrations of thin sea ice, even during winter months. Consolidation of an ice arch at the head of Smith Sound initiates the formation of the polynya, which is further stimulated by northerly winds and currents that remove newly formed sea ice (Ingram et al., 2002; Bi et al., 2019) and sensible heat from WGC upwelling on the Greenland side (Melling et al., 2001; Ingram et al., 2002). Baffin Bay sea ice concentrations decrease between April and August, beginning in the NOW region before propagating southward and creating a generally ice-free Baffin Bay by June (Bi et al., 2019). The Pacific Water, a major component of the ASW, has twice the nitrogen and phosphorus and 7 times the silica of Atlantic Water (Jones et al., 2003). The high nutrient content of incoming ASW, along with higher light levels and stratification in the NOW, fuels high seasonal phytoplankton productivity (Lewis et al., 1996; Ingram et al., 2002; Tremblay et al., 2002). Productivity is an order of magnitude higher in the NOW than in adjacent areas of Baffin Bay, making it one of the most important areas for new production in the Arctic (Tremblay et al., 2002).
Highly branched isoprenoids (HBIs) are unsaturated hydrocarbons (Fig. S1)
biosynthesized by a narrow range of marine diatoms (see review by Belt,
2018). The mono-unsaturated HBI termed IP
Other HBIs, such as the tri-unsaturated isomers HBI III and IV, have been
attributed to biosynthesis by open-water phytoplankton (Belt et al., 2000,
2008, 2015, 2017; Rowland et al., 2001). The PIP
Sterols are ubiquitous components in eukaryotic organisms (Fig. S2,
Volkman, 1986) and, similar to HBI III and IV, have become common
complementary biomarkers in IP
Isoprenoid glycerol dibiphytanyl glycerol tetraethers (GDGTs) are cell-membrane-spanning lipids biosynthesized by archaea (Fig. S3, Pearson and
Ingalls, 2013), including ammonia-oxidizing Thaumarchaeota (Schouten et al.,
2002; Könneke et al., 2005; Pitcher et al., 2011; Besseling et al.,
2020). Thaumarchaeota can modify the number of cyclopentane moieties
(cyclization) in GDGTs in response to in situ temperature variability, a process
known as homeoviscous adaptation (e.g., Elling et al., 2015). Thus,
correlations are found between the degree of GDGT cyclization in global
surface sediment datasets and upper-ocean temperature. These degrees of
cyclization are commonly reported with various versions of the tetraether
index of tetraethers consisting of 86 carbons (TEX
Evidence from a latitudinal transect in the western Atlantic Ocean
demonstrates that GDGTs are most likely produced and exported to the
seafloor from 80–250 m water depth (Hurley et al., 2018), which compares
well to archaea abundance maxima at 200 m water depth in the Pacific Ocean
(Karner et al., 2001). Considering that Thaumarchaeota are
chemolithoautotrophs that perform ammonia oxidation (conversion of ammonia
to nitrite), they are typically more abundant around the primary nitrite
maximum near the base of the photic zone (Church et al., 2010; Francis et
al., 2005; Hurley et al., 2018) and are most productive when there is
minimized phytoplanktonic competition over ammonia (Schouten et al., 2013). In
the higher latitudes, the latter occurs during the less productive dark
winter months when photosynthesis for sea surface species is inhibited,
which may explain the seasonal winter temperature bias of GDGTs observed in
this latitudinal band (Herfort et al., 2006; Rueda et al., 2009;
Rodrigo-Gámiz et al., 2015; Harning et al., 2019). Although the
temperature relationship of TEX
We analyzed marine surface sediment samples (
Marine surface sediment site information for this study.
At the University of Colorado Boulder's Earth Systems Stable Isotope
Laboratory, freeze-dried and decalcified marine surface sediment subsamples
(
At the University of Colorado Boulder's Organic Geochemistry Laboratory,
freeze-dried marine surface sediment subsamples (
From F1, we focus on highly branched isoprenoid (HBI) IP
From F4, we focus on a series of diagnostic sterols, namely brassicasterol
(24-Methylcholesta-5,22
For GDGTs, we focus on isoprenoid and hydroxylated isoprenoid GDGTs. A 25 % aliquot of dry TLE samples was resuspended in hexane : isopropanol
(
For productivity biomarkers (HBIs and sterols), we computed Pearson
correlation matrices and
WOA18 Annual 2007–2017 oceanographic variables from Baffin Bay against depth (m b.s.l.). Individual profiles are from each study site, where darker (lighter) colors reflect sites farther north (south) and dashed (solid) lines denote those within (outside) the modern limits of the NOW. Data from Garcia et al. (2018a, b), Locarnini et al. (2018), and Zweng et al. (2018).
Although only 9 out of the 13 total surface sediment samples were analyzed
for bulk geochemistry, the 9 that were analyzed represent the full spatial
range of our total 13 samples in Baffin Bay (Fig. 1). Bi-plots of
HBIs are present above the detection limit in all sediment samples
(
Sterols are present above the detection limit in all sediment samples
(
We calculated new Baffin Bay balance factors (
Bulk geochemistry data for this study's dataset
(
Average concentrations (ng g SD
GDGTs and OH-GDGTs are present above the detection limit in all sediment
samples (
Pearson correlation coefficients between HBIs and sterols
for
In terms of regressions against different environmental variables (e.g.,
temperature, salinity, dissolved oxygen, and nitrate), we find that the
tested GDGT- (TEX
GDGT- and OH-GDGT distributions and average fractional
abundances and standard deviations for this study's dataset
(
Regression coefficients of GDGT-based temperature indices
against WOA18 temperature at various depth integrations and seasons for this
study's dataset (
For the two indices that feature the strongest significant correlations with
temperature (
GDGT- and OH-GDGT temperature calibrations for this
study's dataset (
A recent and expanded Arctic study by Kolling et al. (2020) explored the
efficacy of using a variety of sedimentary HBIs (i.e., IP
In terms of HBIs, mean concentrations of all compounds are higher in sites
within the NOW compared to sites outside the NOW in both datasets (Fig. 4a–b). However, only the concentration of HBI II is significantly different
between NOW and non-NOW sites across the two datasets (Fig. 4a–b), while HBI
IV is significantly different in this study (Fig. 4a) and HBI III is
significantly different in Kolling et al. (2020) (Fig. 4b). For IP
In terms of sterols, mean concentrations of all compounds were higher and
statistically different for sites in the NOW compared to sites outside the
NOW in both datasets (Fig. 4c–d). We note that the non-NOW data for
brassicasterol and dinosterol from Kolling et al. (2020) are proportionally
higher than ours, which may reflect the incorporation of a greater diversity
of oceanographic settings in Baffin Bay compared to ours. While a recent
Holocene marine record from Petermann Fjord (northwestern Greenland) interprets
campesterol and
Finally, in terms of PIP
In summary, we have several recommendations for future lipid-based
paleoenvironmental reconstructions in Arctic oceanographic settings. First,
for complex oceanographic settings like Baffin Bay, we recommend the
analysis of both sterols and HBIs to test the performance of various sea ice
cover proxies and their ability to track local changes in sea ice and
pelagic productivity. If possible, this would be best achieved through the
analysis of modern surface sediments and sediment traps over an
environmental gradient and through the seasons to capture proxy response to
known variable changes (e.g., Navarro-Rodriguez et al., 2013; Smik and Belt, 2017;
Koch et al., 2020). Second, continued research on sterol and HBI sources and
seasonality of production is critical for the development of more refined
sea ice and marine productivity reconstructions (e.g., Limoges et al., 2018;
Amiraux et al., 2019, 2021). This is particularly necessary when combining
IP
Temperature correlations for GDGTs first relied on empirical correlations
between global surface sediments and the variability in biomarker structure
(e.g., Schouten et al., 2002). Subsequent iterations and developments of
TEX
While a more detailed analysis of intact polar lipid production and genetic
diversity in Baffin Bay is lacking, the distribution of GDGT and OH-GDGT in
our study area (Fig. 6) and understanding of their production in cultures
(Elling et al., 2017) indicate that planktic group 1.1a Thaumarchaeota are
likely the dominant producers. Therefore, the global relationship between
TEX
Our regression analysis against temperature, salinity, DO, and nitrate
further supports temperature as the dominant environmental control on GDGT
distributions in Baffin Bay for the seasons available in the WOA18 dataset.
The lack of WOA18 winter temperatures, in addition to the fragmentary
dataset for spring temperatures in Baffin Bay (Locarnini et al., 2018),
prevents us from assessing the impact of these individual seasons, which is
unfortunate given that cold season temperatures in other high-latitude
settings exhibit a stronger correlation with GDGT distributions (Herfort et
al., 2006; Rueda et al., 2009; Rodrigo-Gámiz et al., 2015; Harning et
al., 2019). However, the higher correlation between TEX
In terms of other tested environmental variables, correlations between
TEX
Our complementary analysis of OH-GDGTs and environmental variables reveals
several differences with the conclusions drawn for GDGTs in Baffin Bay and
with OH-GDGTs elsewhere. First, the RI-OH
In terms of additional environmental variables, only surface dissolved
oxygen appears to exert a partial influence on OH-GDGT cyclization in the
RI-OH index (Fig. S13c). However, when OH-GDGT-0 is considered in the RI-OH
Following our evaluation of GDGT and OH-GDGT indices in terms of their
ability to capture temperature, amongst other environmental variables, we
present two temperature calibrations that can benefit future
paleoceanographic reconstructions from Baffin Bay (Fig. 8). Like other
regional TEX
As global climate change continues, the sustainability of Arctic polynyas is
in jeopardy. While proxy reconstructions of polynyas prior to the
instrumental period can shed light on key climate and environmental
mechanisms that lead to polynya presence or absence, continued development of
our understanding of those proxies is needed. Therefore, we
evaluated a series of lipid biomarkers (HBIs, sterols, GDGTs, and OH-GDGTs)
in surface sediment samples from Baffin Bay to characterize how these
biomarkers capture sea ice and productivity conditions in the North Water
Polynya (NOW) and inform the utility of commonly applied
paleotemperature proxies in Baffin Bay. In conjunction with a similar study
of lipid biomarker productivity proxies from the same region (Kolling et
al., 2020), we draw the following conclusions.
All studied HBIs (IP All studied sterols (brassicasterol, dinosterol, campesterol, and
Application of the GDGT-based TEX Application of the OH-GDGT-based RI-OH index is optimal over RI-OH Future multi-proxy approaches that include (but are not limited to) these
lipid biomarker tool sets in marine sediment core studies from the NOW will
allow for more refined and detailed interpretations on the polynya's past
inception and stability throughout Earth's history.
After acceptance, all data will be stored on the PANGAEA repository and are also available in the Supplement.
The supplement related to this article is available online at:
JS and DJH designed the study, and JS and AEJ funded the study. DJH led the analyses of samples and developed GC-MS methods under the supervision of JS. BH and LW assisted with the extraction and purification of samples. DJH wrote the manuscript with discussion and contribution from all co-authors.
The contact author has declared that none of the authors has any competing interests.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
The authors are grateful to the Inuit of Inuit Nunangat (Canada), the
Kalaalit of Kalaalit Nunaat (Greenland), and the Inughuit of Avanersuarmiut
(western Greenland) for access to their homelands to conduct this research. We
kindly thank the captains, crews, and scientific staffs aboard the 2008 CSS
This project has been supported by the National Science Foundation (grant no. ARN-1804504).
This paper was edited by Helge Niemann and reviewed by Sofia Ribeiro, Darci Rush, and one anonymous referee.