BGBiogeosciencesBGBiogeosciences1726-4189Copernicus PublicationsGöttingen, Germany10.5194/bg-15-5221-2018Carbonate system distribution, anthropogenic carbon and acidification in the western tropical South Pacific (OUTPACE 2015 transect)Carbonate system in the WTSPWagenerThibautthibaut.wagener@univ-amu.frhttps://orcid.org/0000-0002-3968-0678MetzlNicolasCaffinMathieuFinJonathanHelias NunigeSandraLefevreDominiqueLo MonacoClaireRougierGillesMoutinThierryhttps://orcid.org/0000-0003-1297-8893Aix Marseille Univ, CNRS, IRD, Université de Toulon, MIO UM 110, 13288, Marseille, FranceSorbonne Université, CNRS, IRD, MNHN, Laboratoire d'océanographie et du climat: expérimentation et approches numériques (LOCEAN), Case 100, 4 place Jussieu, 75252 Paris CEDEX 05, FranceThibaut Wagener (thibaut.wagener@univ-amu.fr)29August20181516522152369April201817April201824July201827July2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://bg.copernicus.org/articles/15/5221/2018/bg-15-5221-2018.htmlThe full text article is available as a PDF file from https://bg.copernicus.org/articles/15/5221/2018/bg-15-5221-2018.pdf
The western tropical South
Pacific was sampled along a longitudinal 4000 km transect (OUTPACE cruise,
18 February, 3 April 2015) for the measurement of carbonate parameters (total
alkalinity and total inorganic carbon) between the Melanesian Archipelago
(MA) and the western part of the South Pacific gyre (WGY). This paper reports
this new dataset and derived properties: pH on the total scale
(pHT) and the CaCO3 saturation state with respect to
aragonite (Ωara). We also estimate anthropogenic carbon
(CANT) distribution in the water column using the TrOCA method
(Tracer combining Oxygen, inorganic Carbon and total Alkalinity). Along the
OUTPACE transect a deeper penetration of CANT in the intermediate
waters was observed in the MA, whereas highest CANT
concentrations were detected in the subsurface waters of the WGY. By
combining our OUTPACE dataset with data available in GLODAPv2 (1974–2009),
temporal changes in oceanic inorganic carbon were evaluated. An increase of
1.3 to 1.6 µmol kg-1 a-1 for total inorganic carbon in
the upper thermocline waters is estimated, whereas CANT increases
by 1.1 to 1.2 µmol kg-1 a-1. In the MA intermediate
waters (27 kg m-3<σθ<27.2 kg m-3) an
increase of 0.4 µmol kg-1 a-1CANT is
detected. Our results suggest a clear progression of ocean acidification in
the western tropical South Pacific with a decrease in the oceanic
pHT of up to -0.0027 a-1 and a shoaling of the saturation
depth for aragonite of up to 200 m since the pre-industrial period.
Introduction
Human activities inject about 10 petagrams of carbon per year into the
atmosphere, which might have major consequences on climate. It is recognized
that the ocean plays a key role in the control of atmospheric CO2
through uptake by the so-called “oceanic carbon pump”. Through this
“pump”, the ocean sequesters ca. 25 % of the CO2 injected
annually into the atmosphere by human activities .
A consequence of the ocean carbon uptake is a decrease in the oceanic pH
, which is described as ocean acidification (the
so-called “other” CO2 problem). Effects of ocean acidification have
been observed in marine organisms and could affect the marine ecosystems
. Improving our understanding of the oceanic
CO2 uptake relies primarily on observations of the marine carbonate
cycle. Studies on the oceanic carbonate cycle have been mostly conducted in
the framework of international programs. The World Ocean Circulation Experiment
(WOCE) and the Joint Global Flux Study (JGOFS) in the 1990s coordinated
oceanographic cruises along large sections in the ocean to collect samples
through the water column and to perform accurate measurements of carbonate
parameters and ancillary parameters (temperature, salinity, dissolved oxygen,
nutrients, etc.). Since 2000, efforts have been made to revisit oceanic sections according to the WOCE strategy in order to assess oceanic
changes at the scale of a decade. These programs have generated important
databases for oceanic carbonate chemistry e.g., GLODAPv2,.
In order to better assess the role of the ocean for the global carbon cycle,
the concept of oceanic anthropogenic carbon (CANT) has been
introduced and refers to the fraction of dissolved inorganic carbon
(CT) in the ocean that originates from carbon injected into the atmosphere by human
activities since the industrial revolution. As
CANT is not a directly measurable quantity, it can only be
estimated through assumptions that are subject to intense scientific debate
. In particular, it has been recently
recognized that ocean circulation changes drive significant variability in
carbon uptake . Detecting, separating and
attributing decadal changes in the carbonate system (CT and total
alkalinity, AT), CANT and pH in the ocean at global
or regional scales remains challenging.
Map of the OUTPACE cruise transect. Melanesian Archipelago (MA)
stations are indicated by large dark green large dots and the western gyre
(WGY) stations by large dark blue dots. Stations outside of these two areas
are in gray. The station in red corresponds to the station where the deep
cast and intercomparison cast were made. Stations from the GLODAPv2 database
are indicated with small crosses: small green dots correspond to GLODAPv2
stations considered for comparison in the MA area; small blue dots correspond
to GLODAPv2 stations considered for comparison in the WGY area and small gray
dots are the other GLODAPv2 stations considered for comparison.
Within this context, the Pacific Ocean is a particularly challenging area to
study due to its size (ca. one-third of the Earth's and one half of the
oceanic surface). Even if, due to its remoteness from land, it remains
largely underexplored by oceanographic vessels compared to other oceanic
areas, the Pacific Ocean has been covered by cruises along long sections (the
“P sections” from the WOCE program). Most of these sections have been
revisited during the last years
e.g.,. In a recent study
based on repeated sections in the Pacific (P16 at 150∘ W),
observed a significant increase in CANT
in the top 500 m around 10–30∘ S and a local carbon storage
maximum around 20∘ S in recent years (between 2005 and 2014). In
this context, the OUTPACE data presented in this study, associated with
historical observations (since the pioneer 1974 GEOSECS, Geochemical Ocean Sections Program) offer a new view to
evaluate variability and decadal changes in CT, CANT
and pHT in the tropical Pacific, here focusing on the western
tropical South Pacific (WTSP).
The aim of this paper is to report a new dataset of oceanic inorganic carbon
(based on measurements of CT and AT) acquired in the
WTSP during the OUTPACE (Oligotrophic to UltTra oligotrophic PACific
Experiment) cruise performed in 2015 . The main
focus of the OUTPACE cruise was to study the complex interactions between
planktonic organisms and the cycle of biogenic elements on different scales,
motivated by the fact that the WTSP has been identified as a hot spot of
N2 fixation . The data presented here have
been partially used in another paper of this special issue
in order to study the biological carbon pump in
the upper (surface to 200 m) water column. In this paper we will explore the
carbonate data between the surface and 2000 m depth. The OUTPACE transect
(Fig. 1) is close to existing WOCE and GO-SHIP (Global Ocean Ship-based Hydrographic Investigations Program) lines in the South Pacific:
it is parallel to the zonal P21 line (18∘ S visited in 1994 and
2009) and the P06 line (32∘ S visited in 1992, 2003 and 2010), it is
crossed by the meridional P14 line (180∘ E visited in 1994 and 2007)
and P15 line (170∘ W visited in 2001, 2009 and 2016), and it is
situated at the eastern side of the P16 line (150∘ W visited in
1992, 2005 and 2014). However, the OUTPACE transect does not correspond to
any earlier occupation of the “WOCE lines” in the South Pacific and no
tracers of water mass age were measured during the cruise, which limits the
possibilities of a robust analysis of CANT accumulation in the
area. Moreover, the horizontal and vertical resolution of the OUTPACE dataset
is low. In consequence, the OUTPACE dataset cannot be used to look at decadal
changes in CANT content in the South Pacific
e.g.,. Here,
CANT estimates based on the TrOCA (Tracer combining Oxygen,
inorganic Carbon and total Alkalinity) method will be used as as a tool to
investigate changes in CT. Moreover, comparing our data with the
high-quality data (internally consistent through a secondary quality control;
) available in the Global Ocean Data analysis
Project version 2 (GLODAPv2 database) will allow us to evaluate
CT, AT, CANT (from TrOCA) and
pHT trends in subsurface waters and at depth.
General description of the casts sampled for carbonate chemistry
parameters during the OUTPACE cruise.
a SHAW stands for casts up to 200 dbar, INT stands
for casts up to 2000 dbar, DEEP stands for the deep cast and REPRO stands
for the cast with reproducibility measurements. b A CTD was
rosette used for the cast. CLA is the normal CTD rosette, and TMC is the
trace metal clean rosette (see Sect. 2.1).
The paper is organized as follows: after describing the methods used to
acquire the dataset and the way the auxiliary data have been used in
Sect. , we briefly present the hydrographic context of the cruise
in Sect. . We then present in Sect. , the
carbonate dataset acquired during the cruise. In Sect. ,
estimated CANT values in the water column are presented, the
validity of these estimates based on the TrOCA method is discussed and
geographical patterns are evoked. In Sect. , the temporal
changes in oceanic inorganic carbon in the WTSP combining data available in
GLODAPv2 and our OUTPACE dataset are presented and discussed. Finally, in
Sect. , some features in relation to ocean acidification are
inferred from our dataset.
Material and methodsCruise and sampling strategy
The OUTPACE cruise took place between 18 February and 3 April 2015 from
Nouméa (New Caledonia) to Papeete (French Polynesia), in the WTSP on
board the French research vessel L'Atalante (Fig. ). A
total of 18 stations were sampled, mostly in the top 2000 m of the water
column along a ∼4000 km transect from Melanesia to the South Pacific gyre
. A conductivity–temperature–depth
(CTD) rosette was deployed to acquire data with CTD and
associated sensors along vertical profiles and to collect discrete seawater
samples from twenty-four 12 L Niskin bottles for chemical analysis. Due to
technical failures on the main CTD rosette, for two of the casts considered
in this study, a trace metal clean CTD rosette (TM-R) equipped with 24
teflon-lined GO-FLO bottles devoted to trace metal analyses was used. The
configurations of both CTD rosettes are detailed elsewhere
.
For carbonate parameters, seawater was sampled from 37 casts over the
18 stations. At each station, on a regular basis, samples were collected at
12 depths between the surface and 2000 m on two distinct casts: six samples
on a 0–200 m cast and six samples on a 0–2000 m cast. At station SD 13,
only one cast was sampled down to 500 m depth. In addition, at station LD C,
samples were collected at 24 depths on a deep cast (down to 5000 m) and 12
samples were collected at the same depth (25 m) on a “repeatability” cast.
Details on the casts performed for this study are summarized in
Table .
Chemical measurements on discrete samples
All samples were collected within less than 1 h after arrival of the CTD
rosette on deck.
Total alkalinity and dissolved inorganic carbon
Samples for AT and CT were collected in one 500 mL
borosilicate glass flask (Schott Duran®)
and poisoned with HgCl2 immediately after collection (final
concentration 20 mg L-1). Samples were stored at 4 ∘C during
transport and were analyzed (within 10 days of each other) 5 months after the
end of the cruise at the SNAPO-CO2 (Service National d'Analyse des
paramètres Océaniques du CO2 – LOCEAN – Paris).
AT and CT were measured on the same sample based on a
potentiometric titration in a closed cell . A
nonlinear curve fitting approach was used to estimate AT and
CT from the recorded titration data
. Measurements were calibrated with
certified reference material (CRM) provided by Andrew Dickson, University of
Southern California (batch 139 – CT: 2023.23±0.70µmol kg-1 and AT: 2250.82±0.60µmol kg-1; see ). The
reproducibility, expressed as the standard deviation of the CRM analysis
(n=15), was 4.6 µmol kg-1 for AT and
4.7 µmol kg-1 for CT. Based on replicate
measurements at station LD C (cast out_c_194; see Table ), the
reproducibility, expressed as the standard deviation of the analysis of the
replicates collected at the same depth (ca. 25 m, n=12) from different
Niskin bottles, was 3.6 µmol kg-1 for AT (average
value:2324.7µmol kg-1) and
3.7 µmol kg-1 for CT (average value:1969.7µmol kg-1).
Oxygen concentration
Dissolved oxygen concentration [O2] was measured following the
Winkler method with potentiometric endpoint
detection . For sampling, reagent preparation and
analysis, the recommendations from were
carefully followed. The thiosulfate solution was calibrated by titrating it
against a potassium iodate certified standard solution of 0.0100 N
(CSK standard solution –
WAKO™). The reproducibility, expressed as
the standard deviation of replicates samples was
0.8 µmol kg-1 (n=15; average value:195.4µmol kg-1).
Vertical profiles of hydrological and biogeochemical parametersCTD measurements
CTD measurements were ensured by a Seabird™
911+ underwater unit, which interfaced an internal pressure sensor, two
redundant external temperature probes (SBE3plus) and two redundant external
conductivity cells (SBE4C). The sensors were calibrated pre- and post-cruise
by the manufacturer. No significant drift between the redundant sensors was
observed. For vertical profiles, full-resolution data (24 Hz) were reduced
to 1 dbar binned vertical profiles on the downcast with a suite of
processing modules using the Seabird™
dedicated software (SbeDataProcessing). For values at the closure of
the Niskin bottles, values collected at 24 Hz were averaged 3 s before and
5 s after closure of the bottle. In this study, for temperature and
conductivity the signal of the first sensors has been used systematically. For
the two TM-R casts, no significant difference with the main CTD rosette on
temperature and conductivity was observed.
Oxygen measurements
[O2] was also measured with a SBE43 electrochemical sensor
interfaced with the CTD unit. The raw voltage was converted to oxygen
concentration with 13 calibration coefficients based on the
Seabird™ methodology derived from
. Three of these coefficients (the oxygen signal slope,
the voltage at zero oxygen signal, the pressure correction factor) were
adjusted with the concentrations estimated with the Winkler method on samples
collected at the closure of the bottles. One unique set of calibration
coefficients has been applied to all oxygen profiles from the cruise because
no significant drift of the sensor was observed during the time of the
cruise. For the two TM-R casts, values have been corrected with a drift and
offset based on the comparison of 15 pairs of casts (main CTD rosette/TM-R)
collected close in time (less than 2 h) and space (less than 1 nautical
mile) over the entire OUTPACE transect.
Derived parameters
Practical salinity (SP) was derived from conductivity,
temperature and pressure with the EPS-78 algorithm. Absolute salinity
(SA), potential temperature (θ), conservative temperature
(Θ) and potential density (σθ) were derived from
SP, temperature, pressure and the geographic position with the
TEOS-10 algorithms . These five derived
parameters were calculated within the processing with
Seabird™ dedicated software.
Seawater pH on the total scale (pHT) and the CaCO3
saturation state with respect to aragonite (Ωara) were
derived from AT and CT with the “Seacarb” R package
. CaCO3 saturation state with respect
to calcite was not considered because seawater up to 2000 dbar was
supersaturated with respect to calcite (Ωcal>1). Following the recommendations from
, the constants for carbonic acid K1 and
K2 from , the constant for hydrogen fluoride
KF from , and the constant for hydrogen
sulfate KS from were used.
Orthophosphate and silicate concentration were considered in the calculation.
Methods for nutrient measurement are presented in detail in
. When nutrient data were not available (station
SD 8), silicate and orthophosphate were estimated from the nutrient profile
measured on cast out_c_163 (interpolated values). Apparent oxygen
utilization (AOU) was computed from the difference between oxygen solubility
(at p=0 dbar, and SP) estimated with the “Benson and Krause
coefficients” in and in situ [O2].
For the estimation of CANT, the TrOCA method was used. The TrOCA
approach was first proposed in
with improvements
in . In brief, the TrOCA parameter is defined as
a combination of AT, CT and [O2] that
accounts for biologically induced relative changes among these parameters
(with constant stoichiometric ratios). TrOCA is thus a quasi-conservative
tracer derived from CT in the ocean. Within a defined water mass,
changes in TrOCA over time are independent of biology and can be attributed
to the penetration of CANT. In consequence CANT can
be calculated in a parcel of water from the difference between current and
pre-industrial TrOCA (TrOCA∘) divided by a stoichiometric
coefficient. The simplicity of the TrOCA method relies on the fact that a
simple formulation for TROCA∘ has been proposed based on potential
temperature and alkalinity and thus an estimation of CANT can be
done by a simple calculation using CT, AT,
[O2] and θ. In this study, the formulation proposed in
Eq. (11) in is used to calculate
CANT and is recalled here in Eq. ().
CANT=([O2]+1.279CT-AT2-exp(7.511-1.087×10-2θ-7.81×10-5AT2))/1.279
This formulation is based on an
adjustment of the TrOCA coefficients using Δ14C and CFC-11
from the GLODAPv1 database .
estimated the overall uncertainty of the
CANT with TrOCA method to be ca. 6 µmol kg-1
based on the random propagation of the uncertainties in the variables
(CT, AT, [O2] and θ) and coefficients
used in Eq. (). The limitations and validity of the TrOCA method
will be discussed in detail in Sect. .
Data from available databases
For comparison with existing values of carbonate chemistry in the area of the
OUTPACE cruise, relevant data were extracted from GLODAPv2 database (NDP-93
– ). The specific data file for
the Pacific Ocean was used (downloaded from
https://www.nodc.noaa.gov/ocads/oceans/GLODAPv2/, last access:
14 December 2017). For comparison with OUTPACE data, GLODAPv2 data were
selected between 22 and 17∘ S and between 159∘ E and
159∘ W (going westwards). For specific comparisons in the
MA and the South Pacific western
gyre waters (WGY) a zonal subset of the extracted data was used:
159∘ E and 178∘ W for MA and 170 to 159∘ W for WGY
(see Fig. ).
Hydrological context along the OUTPACE transect
The hydrological context encountered during the OUTPACE transect is presented
with a Θ-SA diagram between 0 and 2000 dbar in Fig. . A detailed description of the water masses encountered
during the OUTPACE cruise can be found in .
Briefly, from the surface to 2000 dbar, the following features are
distinguished: the surface waters (σθ<23.5 kg m-3)
were characterized by temperatures over 25 ∘C with increasing
temperature and salinity towards the east and AOU close to zero. Under
the surface water, the upper thermocline waters (UTW) presented a maximum in
salinity, reaching values higher than 36 g kg-1 in the eastern part of
the cruise. In the lower thermocline waters, SA decreased with
depth with a more pronounced decrease in the eastern part than in the western
part, whereas AOU is higher in the eastern part than in the western part of
the studied area. These differences in lower thermocline waters have been
described for South Pacific Central Waters (SPCW) with more saline western
(WSPCW) and less saline eastern (ESPCW) waters .
Below the thermocline, intermediate waters are made up of Subantarctic
Mode Waters (SAMW) and Antarctic Intermediate Waters (AAIWs). AAIWs have a
salinity minimum close to the σθ=27 kg m-3 isopycnal.
defines SAMW as having σθ values
between 26.80 and 27.06 kg m-3, corresponding to a minimum in potential
vorticity, and AAIW as having σθ values between 27.06 and
27.40 kg m-3. The separation of both waters is not trivial in the
subtropical area. SAMW is generally associated with lower AOU than AAIW.
Finally, deep waters made up of Upper Circumpolar Deep Waters (UCDW)
correspond to an increase in salinity and AOU for depth corresponding to
σθ>27.4 kg m-3.
Θ-SA diagram with colors indicating the AOU.
Black contour lines represent the isopycnal horizons based on potential
density referenced to a pressure of 0 dbar (σθ).
Longitudinal variations in (a)AT,
(b)CT, (c) pHT and
(d)Ωara along the OUTPACE transect between the
surface and 2000 dbar depth. Black contour lines represent the isopycnal
horizons based on potential density referenced to a pressure of 0 dbar.
Vertical profiles of (e)AT,
(f)ATn35 and
(g)CT of the entire OUTPACE dataset (red dots)
superimposed on the GLODAPv2 data corresponding to the OUTPACE area (gray
dots).
In this study, discussion will sometimes make a distinction between two
subregions along the OUTPACE transect: MA and WGY (see
Sect. for definition). This distinction is mainly based on
geographic and oceanographic arguments. Indeed, these two subregions are
geographically separated by the Tonga volcanic arc. WGY is characterized by
higher surface temperature and a higher salinity in the upper thermocline
waters than MA. The difference between these subregions is evidenced by the
difference in oligotrophy . Due to specific
conditions in the transition area between the MA and WGY
, SD 11, SD 12 and LD B were discarded from both
groups in this study following the arguments in .
Carbonate chemistry along the OUTPACE transect
AT and CT measured along the OUTPACE transect are
presented in Fig. a and b. All vertical profiles for
AT, AT normalized to SA=35 g kg-1 (ATn35) and CT are presented in
Fig. e, f and g. AT ranged between 2300 and
2400 µmol kg-1. Below the surface, a pronounced maximum in
AT was observed associated with the saltier upper thermocline
waters. When normalized to SA=35 g kg-1,
ATn35 values are remarkably constant in the upper 500 dbar
with values between 2270 and 2310 µmol kg-1. Below 500 dbar,
AT increases with depth up to ca. 2400 µmol kg-1
indicating that alkalinity changes are mostly due to salinity changes in the
upper water column, whereas the increase in the deep waters is mainly due to
carbonate biomineral remineralization. CT values are close to
1950 µmol kg-1 in the surface and increase with depth up to
2300 µmol kg-1 at 2000 dbar. The CT gradient in
the upper water column has been described in .
Below 2000 dbar, CT is relatively invariant with slightly lower
values in the bottom waters (below 4000 dbar) due to the presence of very
old deep waters originating from the North Pacific relative to the northward
moving bottom waters that have not accumulated as much carbon
. AT and CT values in deep
waters measured during OUTPACE are in good agreement with the data of the
GLODAPv2 database (Fig. e, f and g). No systematic adjustment of
the OUTPACE dataset with the GLODAPv2 dataset was performed because only very
few data are available in the deep ocean where crossover comparison can be
performed for cruises carried out in different decades. Nevertheless, for the
only “deep” cast performed during OUTPACE (out_c_163 at station LD C), we
performed a simple crossover analysis with the station 189 (located at
107 km kilometers from OUTPACE station LD C) of the Japanese “P21
revisited” cruise in 2009. We compared interpolated profiles on density
surfaces values (27.75 kg m-3<σθ<27.83 kg m-1 corresponding to pressure levels of ca. 3000 to
5500 dbar). The estimated offsets are -2.0±4.2µmol kg-1 for AT and -2.0±4.4µmol kg-1 for CT suggesting measurement
biases are likely no larger. This simple quality control procedure seems to
indicate that no systematic adjustment is needed.
Derived parameters from the AT and CT measurements
are presented in Fig. c for pHT values (estimated at in
situ temperature and pressure). pHT decreases from values close
to 8.06 in surface to values close to 7.84 at 2000 m. Surface values of
pHT are typical of subtropical warm waters and are in a similar
range as the austral summer values estimated by
in this area (8.06–8.08).
Figure d represents the vertical distribution of computed values of
Ωara along the OUTPACE transect. Seawater is supersaturated
with respect to aragonite (Ωara>1) at the surface with
Ωara values of ca. 4.0 again in good agreement with the
austral summer values of 4–4.4 estimated by
in this area. Values of
Ωara decrease with depth, and seawater becomes undersaturated
with respect to aragonite (Ωara<1) at an horizon situated
below 1000 dbar in the west and above 1000 dbar in the eastern part of the
cruise, with a general shoaling of the Ωara values from west
to east, in good agreement with a previous study by
in this area.
Anthropogenic carbon estimation along the OUTPACE transect
The TrOCA method is a way to quantify CANT in the ocean based on
CT, AT, [O2] and θ. This method has
been used and compared to other methods in different oceanic areas (e.g.,
;
). Based on specific
CANT inventories in the water column, the TrOCA method reasonably
agreed with the other methods (including the transient tracer-based method).
However, “tested” the TrOCA method within an
ocean general circulation model and argued that the use of globally uniform
parameterization for the estimation of the pre-industrial TrOCA is a source
of significant overestimation but also that even with regionally “tuned”
parameters a global positive bias in the method exists. As no tracers of
water mass age were measured during the OUTPACE cruise, the main motivation
for using the TrOCA method was to make CANT estimations based on
a simple calculation from parameters acquired within the cruise as done in
other cruises conducted in tropical South Pacific waters
e.g.,. Even if
CANT estimates from TrOCA are biased, the application of a simple
back-calculation method that accounts for biologically induced relative
changes in CT is used here to identify some spatial features in
the distribution of the carbonate system along the OUTPACE transect. Here, an
error in the TrOCA CANT estimates of 67 % will be considered
based on the standard deviation for the TrOCA variant optimized with world
ocean data and normalized such that standard deviation in the simulated
CANT in the ocean general circulation model is exactly 1 (see
Table 2 in ).
Longitudinal variations in CANT (estimated with the
TrOCA method) along the OUTPACE transect between 100 and
2000 dbar depth (a).
Black contour lines represents the isopycnal horizons based on potential
density referenced to a pressure of 0 dbar. Vertical profiles of
CANT for the entire OUTPACE dataset superimposed on the values
estimated from the GLODAPv2 data (b) and vertical profiles of
CANT between 100 and 1500 dbar superimposed on the values estimated from the recent (after 2005)
GLODAPv2 data (c). The color code for the dots is the same as for
Fig. .
As mentioned by , CANT estimates
cannot be considered within the mixed layer because the underlying hypotheses
used in the formulation of TrOCA may not be verified due to biological
activity and gas transfers across the air–sea interface. To avoid this
issue, CANT estimates are generally used below the “permanent”
mixed layer depth e.g.,.
For the OUTPACE area, show that the mixed layer
depth does not exceed 70 m in the area. Even if the depths of the deep
chlorophyll maximum were encountered below 100 dbar along the transect, we
will consider CANT values up to 100 dbar. It can be mentioned
that the CANT values of 50–60 µmol kg-1 in the
top of the water column (100 dbar) are in reasonable agreement with a rough
estimate of thermodynamically consistent CT changes: by assuming that
CO2 in surface seawater is in equilibrium with the atmosphere, we
estimated that with a partial pressure of CO2 (pCO2)
of 280 µatm in the pre-industrial period, a pCO2
of 380 µatm during OUTPACE and a
constant AT over time of 2300 µmol kg-1,
CT change in surface waters between the pre-industrial period and 2015 is ca. 65 µmol kg-1 for a temperature of surface waters between
25 and 28 ∘C. For OUTPACE, CANT estimates below
1000 dbar were not significantly different from 0 µmol kg-1, with a standard deviation of 6.3 µmol kg-1.
Temporal evolution in the OUTPACE area of CT(a, d), CANT(b, e) and pHTinsi(c, f)
based on GLODAPv2 and OUTPACE data along two isopycnal layers:
25–25.5 kg m-3(a, b, c) and
27–27.2 kg m-3(d, e, f). The color code for the dots is the
same as for Fig. .
CANT distribution along the OUTPACE transect is presented in Fig. a, and all vertical profiles for CANT are presented
in Fig. b with a more detailed view of the first 1500 dbar of the
water column in Fig. c. Figure b and c distinguish values
from the MA and the WGY area. The CANT vertical profiles suggest
a penetration of anthropogenic carbon up to 1000 dbar. As mentioned before,
estimated values of CANT reach values of 60±40µmol kg-1 at a depth of 100 dbar; CANT then regularly decreases
to values close to 10–20±13µmol kg-1 at a depth of
1000 dbar and reaches values close to 0 µmol kg-1 below
1500 dbar. The zonal CANT section along the OUTPACE transect
(Fig. a) presents two features: (1) a deeper penetration of
CANT in the western part of the transect with values of
CANT reaching 40±25µmol kg-1 around the
isopycnal layer of 27 kg m-3 (ca. 700 dbar) with a coherent behavior
with the distribution of AOU and (2) a larger accumulation of
CANT in the eastern part of the transect centered around the
isopycnal layer of 25 kg m-3 (ca. 200 dbar).
Several studies have identified deeper CANT penetration in the
western South Pacific than in the eastern South Pacific at tropical and
subtropical latitudes. The primary reason for this longitudinal difference
might be associated with deeper convection in the western part and upwelling
in the eastern part. AAIW has been described as the lower limit of the
penetration of CANT in the ocean interior of the South Pacific
. Moreover, a recent study by
shows that ocean circulation variability
is the primary driver for changes in oceanic CO2 uptake at decadal
scales. Based on CT changes between the two repeated visits of
the longitudinal P21 line (18∘ S close to the OUTPACE transect) in
1994 and 2009, show a faster increase in
CANT in the western part than in the eastern part of the section.
They also postulate that CANT may have been transported by deep
circulation associated with the AAIW. In the subtropical Pacific along the
P06 line (longitudinal section at ca. 32∘ S),
also identified an increase in CANT
in the SAMW and AAIW. attribute the deeper
penetration of CANT in the western part of the section to the
local formation of subtropical mode water in the area based on the extended
multiple linear regression (eMLR) method along the P06 line (and taking into
account a third visit).
Estimated trends on AT, [O2], CT ,
CANT and pHT changes in two different layers of the
water column defined by isopycnal layers between 1980 and 2015 based on
GLODAPv2 with (column WITH) and without (column WITHOUT) OUTPACE data added.
Estimated trends are obtained from slope values of a linear regression
between the studied parameters and time.
25 kg m-3<σθ<25.5 kg m-327 kg m-3<σθ<27.2 kg m-3WITHWITHOUTWITHWITHOUTTrend on AT in µmol kg-1 a-1OUTPACE-0.20±0.07 (n=167)∗-0.30±0.07 (n=142)∗-0.12±0.07 (n=180)-0.01±0.06 (n=174)MA-0.30±0.09 (n=85)∗-0.47±0.10 (n=70)∗-0.16±0.09 (n=99)-0.10±0.09 (n=92)WGY-0.20±0.14 (n=28)-0.20±0.19 (n=22)-0.20±0.14 (n=35)-0.01±0.13 (n=31)Trend on [O2] in µmol kg-1 a-1OUTPACE-0.31±0.10 (n=167)∗-0.61±0.09 (n=143)∗0.05±0.11 (n=183)0.07±0.10 (n=178)MA-0.35±0.16 (n=84)∗-0.78±0.17 (n=70)∗0.06±0.11 (n=99)0.04±0.11 (n=93)WGY-0.38±0.11 (n=27)∗-0.35±0.14 (n=23)∗-0.11±0.30 (n=38)-0.22±0.29 (n=34)Trend on CT in µmol kg-1 a-1OUTPACE1.32±0.13 (n=174)∗1.63±0.13 (n=149)∗0.23±0.13 (n=189)0.27±0.11 (n=183)∗MA1.38±0.21 (n=85)∗1.87±0.21 (n=70)∗0.31±0.16 (n=100)0.44±0.17 (n=93)∗WGY1.57±0.18 (n=31)∗1.57±0.23 (n=25)∗0.23±0.29 (n=40)0.23±0.29 (n=36)Trend on CANT in µmol kg-1 a-1OUTPACE1.12±0.07 (n=166)∗1.25±0.06 (n=142)∗0.32±0.05 (n=179)∗0.25±0.04 (n=174)∗MA1.18±0.08 (n=84)∗1.31±0.08 (n=70)∗0.40±0.06 (n=98)∗0.40±0.06 (n=92)∗WGY1.20±0.09 (n=28)∗1.18±0.10 (n=22)∗0.13±0.09 (n=35)0.11±0.08 (n=31)Trend on pHTINSI in a-1OUTPACE-0.0022±0.0003 (n=167)∗-0.0031±0.0002 (n=142)∗-0.0001±0.0003 (n=181)-0.0002±0.0002 (n=175)MA-0.0022±0.0004 (n=85)∗-0.0033±0.0004 (n=70)∗-0.0004±0.0003 (n=100)-0.0007±0.0003 (n=93)∗WGY-0.0027±0.0004 (n=28)∗-0.0030±0.0004 (n=22)∗-0.00008±0.0006 (n=35)-0.0007±0.0006 (n=31)
∗ Trend significant (p level<0.05).
In the eastern part of the OUTPACE cruise, the detected accumulation of
CANT in the upper thermocline waters may be related to recent
observations of a significant accumulation of CANT at latitudes
around 20∘ S on the P16 meridional transect along 150∘ W by
. This change in CANT accumulation is
attributed to changes in the degree of the water mass ventilation due to
variability in a southern Pacific subtropical cell. Along the P16 line,
observed high values of CANT (up
to 60 µmol kg-1) for the upper water column at the latitude
of the OUTPACE area in good agreement with our estimates in WGY in the upper
water column. Finally, it should also be mentioned that, due to the presence
of one of the main oxygen minimum zone (OMZ) area, denitrification occurs in
the eastern South Pacific and can be traced by the N* parameter
. Denitrification, by transforming organic carbon
to inorganic carbon without the consumption of oxygen, could induce an
overestimation of CANT by the TrOCA method (and other
back-calculation methods) due to a biological release of CT that
is not taken into account in the formulation of the quasi-conservative TrOCA
tracer. Horizontal advection by the south equatorial current of the strong
negative N* signal originating from the eastern Pacific towards the
western Pacific has been described previously
. have estimated
N* along the OUTPACE transect and show slightly negative N* values in
the upper thermocline waters at the eastern side of the OUTPACE transect
where the highest CANT values are estimated. However,
showed that, based on a direct relation between
CT and N*, the influence of denitrification should be
negligible on CANT estimations in this area. Therefore, the N*
correction has not been introduced in the CANT estimates and the
effect of denitrification was not quantified here.
Temporal changes in carbonate chemistry in the OUTPACE area
Based on the available GLODAPv2 data, temporal changes in the OUTPACE area
have been assessed (Fig. and Table ). The variation in
oceanic parameters with time is estimated on two isopycnal layers: a layer
with 25 kg m-3<σθ<25.5 kg m-3 (hereafter named
σθ25) and a layer with 27 kg m-3<σθ<27.2 kg m-3 (hereafter named σθ27). These two layers
correspond to the features in CANT discussed in the previous
section. σθ25 can be considered characteristic of the upper
thermocline waters (core of the salinity maximum, Fig. ), whereas
σθ27 can be considered characteristic of intermediate waters
of southern origin (core of the salinity minimum). All the values associated
with these two layers are spread between 145 and 301 dbar for
σθ25 and between 571 and 896 dbar for σθ27. It
must be mentioned that the study of temporal changes is based on a large
sampling grid, which covers the entire OUTPACE transect (see
Sect. and Fig. ). This could add a spatial
variability that may interfere in the estimation of temporal changes.
Estimated depth of the Ωara=1 horizon along the
OUTPACE cruise (see text for details). No values are available for stations
where data up to 2000 dbar were not available (SD2 and SD13). No values were
estimated for stations with CANT<-6µmol kg-1.
StationLongitude (∘ E)Latitude (∘ N)Depth of the Ωara=1 horizon (in m) OUTPACEPre-ind.Difference∗SD 1159.9425-17.90881225NANASD 2162.1248-18.6078NANANASD 3165.0082-19.4907928NANALD A164.5787-19.223310321185153SD 4168.0157-19.9810291193164SD 5169.9965-21.999711261256130SD 6172.1193-21.375810971233136SD 7174.2512-20.767710151235220SD 8176.364-20.694510101171161SD 9178.6087-20.99631214NANASD 11-175.6475-20.005710551172117SD 12-172.7813-19.53681013111299LD B-170.7385-18.1745948104698SD13-169.0728-18.2007NANANALD C-165.7792-18.484285494187SD 14-162.9992-18.39528891006117SD 15-159.9913-18.26189171043126
∗ Difference (in m) between the depth of the
Ωara=1 horizon at the pre-industrial period and the
OUTPACE cruise. NA – not available.
Temporal variations in CT and CANT between 1970 and
2015 are presented in Fig. . As mentioned earlier, even if
CANT estimates from TrOCA are biased, a previous study by
suggests that the TrOCA method gives similar values
to other methods for estimating CANT accumulation rates. A linear
fit was applied to the observed temporal variations for AT,
[O2], CT and CANT to check for significant
trends on data collected between 1980 and 2015. The results of the performed
regression analyses are presented in Table . Trends are evaluated
with and without the data of the OUTPACE cruise in order to estimate the
influence of this new dataset on the observed trends. Trends are evaluated
for the entire OUTPACE area and for the MA and the WGY area. Even if
presented in Fig. , data collected before 1980 from the GLODAPv2
database are disregarded in the estimation of the temporal trends. Indeed,
for the OUTPACE area, data prior to 1980 originate from one single GEOSEC
cruise in 1974, with only one measured point for σθ27 at WGY
and no points at σθ25 for WGY and MA.
At σθ25, a significant decrease in AT of -0.20±0.07µmol kg-1 a-1 is observed over the entire
OUTPACE area. A decrease of -0.30±0.09µmol kg-1 a-1 is also observed in the MA area,
whereas no significant trend is observed for the WGY area. However, when
AT is normalized to salinity, no significant trends are observed
in ATn35 suggesting that the observed trend in AT
can be attributed to salinity changes rather than changes in calcification.
Significant negative trends are observed for [O2] over the entire
area (-0.31±0.10µmol kg-1 a-1), in MA (-0.35±0.16µmol kg-1 a-1) and in WGY (-0.38±0.11µmol kg-1 a-1). The decrease in [O2]
which corresponds to a positive trend in AOU suggested an increase in the
remineralization of organic matter at σθ25. Significant
increasing trends were observed for CT over the entire area
(+1.32±0.13µmol kg-1 a-1), in MA (+1.38±0.21µmol kg-1 a-1) and in WGY (+1.57±0.13µmol kg-1 a-1). For CANT, the trends
were slightly slower (+1.12±0.07 to +1.2±0.09µmol kg-1 a-1) and not significantly different
between MA and WGY. Taking into account the OUTPACE dataset does not change
the overall significance of the observed trends and only minor changes
(mostly within the error of the estimates) are observed. If we assume a
CT increase of 0.5 to 1 µmol kg-1 a-1
(depending on the buffer factors considered) associated with the recent rise in
atmospheric CO2 (see for example ), the
CT increase in the OUTPACE area is faster than thermodynamics
would govern, whereas the CANT is closer to this thermodynamic
value. The higher increase in CT could be related to an increase
in remineralization processes as deduced from [O2] trends, with an
overall consistency between the rate of CT increase and the rate
of decrease in [O2]. However, the important increase in
CANT observed between 2005 and 2015 between 10 and 30∘ S
on the P16 line (at the eastern side of the OUTPACE transect) by
is not supported by significant differences in the
trends of CANT observed between MA and WGY in this study.
At σθ27, the only significant trend observed is an increase in
CANT of ca. 0.40±0.06µmol kg-1 a-1
in the MA area. When the OUTPACE dataset is not considered, a similar trend
is observed for CT in the MA area. This trend is compatible with
the observed increase in CANT by Kouketsu et al. (2013) along the
P21 line close to the isopycnal layer 27 kg m-3. As this increase is
not observed in WGY and if we assume that the σθ27 is filed
with AAIWs, this suggest that the accumulation of CANT in
AAIW is faster west of the 170∘ W line than to the east, but no clear
explanation for this trend can be given.
Longitudinal variations in (a) pHT changes and
(b)Ωara changes between the pre-industrial period
and the present time along the OUTPACE transect between 100 and 2000 dbar
depth (see text for details). Black contour lines represent the isopycnal horizons based on
potential density referenced to a pressure of 0 dbar.
Towards an enhanced “ocean acidification”<?xmltex \hack{\break}?> in the WTSP?
Temporal variations in pHT between 1970 and 2015 are presented in
Fig. c and f with rates of pHT decrease of -0.0022±0.0004 a-1 for MA and -0.0027±0.0004 a-1 for WGY at
σθ25 (Table ) between 1980 and 2015. Based on the
CANT rates estimated in the previous section (1.1 to
1.2 µmol kg-1 a-1) and based on a constant value of
AT of 2285 µmol kg-1 (mean value of
ATn35 on σθ25) and a constant temperature of
20 ∘C (mean value of temperature on σθ25), we can
estimate a pHT decrease rate of -0.0023 to -0.0025 a-1.
This indicates that rates of oceanic pHT decrease (ocean
acidification) can mostly be explained by the increase in CANT.
These rates of acidification are higher than the values reported by
in the western South Pacific along the P06 Line
(south of OUTPACE area at 32∘ S) between two visits in 1992 and
2008. They are also higher than the surface rates of pHT decrease
of -0.0016±0.0001 a-1 recorded at the HOT station in the tropical
North Pacific and of -0.0017±0.0001 and -0.0018±0.0001 a-1
in the tropical North Atlantic at BATS and ESTOC stations, respectively
. However, differences in buffer factors
between the surface and subsurface can partially explain these differences.
Nevertheless, our results in the subsurface (σθ25) based on
GLODAPv2 and OUTPACE data (CT and AT) are similar to
pHT trends derived from fCO2 surface
observations e.g.,. In the southern
subtropical and equatorial Pacific regions, using SOCAT version2,
evaluate contrasting fCO2 and
pHT trends, ranging between +1.1 and
+3.5µatm a-1 for fCO2 and between
-0.001 and -0.0023 a-1 for pHT. If we revisit these
estimates, using surface fCO2 observations available in the
OUTPACE region (18–22∘ S/170–200∘ E) in SOCAT version 6
(; http://www.socat.info, last
access: 10 August 2018) and assuming a constant alkalinity
(2300 µmol kg-1, average of surface data), we can calculate
pHT and CT from fCO2 and temperature
data. The resulting long-term trends for the period 1980–2016 for
fCO2, CT and pHT are, respectively,
+1.27±0.01µatm a-1, +1.03±0.01µmol kg-1 a-1 and -0.0013±0.0001 a-1.
Interestingly, for the period 2000–2016 the trends are +2.53±0.02µatm a-1, +2.02±0.02µmol kg-1 a-1 and -0.0025±0.0003 a-1,
suggesting an acceleration of the signals in recent years. These results,
based on fCO2 observations in surface waters, confirm the
trends we detected for CT and pHT in subsurface
layers (σθ25).
In Fig. , estimates of the so-called “anthropogenic
pHT change” (ΔANTpHT) and
“anthropogenic Ωara change” (ΔANTΩara), which correspond to the difference in pHT
and Ωara between the time of the OUTPACE cruise (modern
time) and the pre-industrial period, are presented. The pHT and
Ωara correspond to the values presented in Fig. ,
whereas the pre-industrial values corresponds to pHT and
Ωara estimated with CT minus CANT.
All other parameters (temperature, salinity, alkalinity and nutrients) are
assumed to remain constant over time. The main features for the distribution
of ΔANTpHT and
ΔANTΩara logically reflect the distribution
of the estimated CANT in this study because CANT is
the only driving force in these estimations. The estimated pHT
decrease reaches values slightly higher than 0.1 and the estimated
Ωara decrease reaches values of 0.75 since the
pre-industrial period for areas with the highest CANT
accumulation. When considering an error in CANT of
6 µmol kg-1, we can assume that we are able to distinguish
changes of 0.0012 for pHT and 0.06 for Ωara.
Decreases in pHT and Ωara are thus detectable
below 1000 dbar in the MA waters and above 1000 dbar in WGY waters.
A decrease in pHT of 0.1 units since the pre-industrial period is
a generally accepted value for oceanic waters affected by CANT
penetration e.g.,. Several
studies have assessed the rate of ocean acidification based on successive
visits to different oceanic areas. For the South Pacific Ocean,
reports decreases in oceanic pHT since
the pre-industrial period of -0.09 and -0.11 pHT units for
the latitude band from 10 to 20∘ S and from 20 to 30∘ S,
respectively, along the P16 line (150∘ W) situated on the eastern
side of the OUTPACE area. These are in good agreement with our estimates in
this area.
Based on an interpolation of the estimated Ωara during
OUTPACE and the pre-industrial Ωara, we calculated the depth
of the horizon where Ωara=1 for the different stations of
the OUTPACE transect (Table ) in 2015 and the pre-industrial period
based on the ΔANTΩara estimates. We observed
an upward migration of the aragonite saturation horizon of up to 220 m in
the MA area along the OUTPACE transect (Table ). This upward
migration of the Ωara=1 horizon is higher than the
migration of 30 to 100 m observed between the 1990s and the pre-industrial period in early studies
in the Pacific based on the WOCE dataset
illustrating the continuous acidification of the WTSP.
Conclusions
Based on AT and CT data and related properties
collected during the OUTPACE cruise, we estimated different parameters of the
carbonate system along a longitudinal section of nearly 4000 km and up to
2000 dbar in WTSP. Even if the vertical and horizontal resolution is low
compared to the WOCE lines and precludes a rigorous comparison with this high-quality dataset, we estimated that the measured carbonate chemistry
parameters are in good agreement with previous data collected in this area.
Based on the estimation of CANT from the TrOCA method, we find
CANT penetration in the WTSP and impacts on pHT and
saturation state of calcium carbonate since the pre-industrial period that
are in good agreement with previous observations in this area. As mentioned
above, CANT values from TrOCA estimates are not reliable in surface
layer. However, based on GLODAPv2 and the SOCAT database, our estimation of
CANT in the subsurface seems to be in good agreement with expected
changes in surface waters. The enhanced impact of ocean acidification in the
subtropical South Pacific suggested by our study highlights the necessity of
sustained research efforts in this largely underexplored part of the world ocean. The presented dataset collected along the OUTPACE transect could
complement existing sections visited nearly every decade in the South Pacific
Ocean and in particular the P21 line, which was last visited in 2009.
OUTPACE cruise data are available at the French INSU/CNRS
LEFE CYBER database (scientific coordinator: Hervé Claustre; data manager
and webmaster: Catherine Schmechtig) at the following web address:
http://www.obs-vlfr.fr/proof/php/outpace/outpace.php (last access:
10 August 2018). GLODAPv2 data are available at the following web address:
https://www.glodap.info/ (last access: 10 August 2018). SOCAT version 6
data are available at the following web address:
https://www.socat.info/ (last access: 10 August 2018).
The authors declare that they have no conflict of
interest.
This article is part of the special issue “Interactions between
planktonic organisms and biogeochemical cycles across trophic and N2
fixation gradients in the western tropical South Pacific Ocean: a
multidisciplinary approach (OUTPACE experiment)”. It is not associated with
a conference.
Acknowledgements
This is a contribution of the OUTPACE (Oligotrophy from Ultra-oligoTrophy
PACific Experiment) project (https://outpace.mio.univ-amu.fr/, last
access: 10 August 2018) funded by the French national research agency
(ANR-14-CE01-0007-01), the LEFE-CyBER program (CNRS-INSU), the GOPS program
(IRD) and the CNES (BC T23, ZBC 4500048836). The OUTPACE cruise
(10.17600/15000900) was managed by the MIO (OSU Institut Pytheas, AMU)
from Marseille (France), which has received funding from the European FEDER
Fund under project 1166-39417. The SNAPO-CO2 service at LOCEAN is
supported by CNRS-INSU and OSU Ecce-Terra. The Surface Ocean CO2
Atlas (SOCAT) is an international effort, endorsed by the International Ocean
Carbon Coordination Project (IOCCP), the Surface Ocean Lower Atmosphere Study
(SOLAS) and the Integrated Marine Biosphere Research (IMBeR) program, to
deliver a uniformly quality-controlled surface ocean CO2 database.
The many researchers and funding agencies responsible for the collection of
data and quality control are thanked for their contributions to SOCAT. The
authors thank the crew of the R/V L'Atalante for outstanding
shipboard operation. Catherine Schmechtig is warmly thanked for the LEFE
CYBER database management. Aurelia Lozingot is acknowledged for the
administrative work. Pierre Marrec is thanked for his insightful comments on
the present work. The two anonymous referees are thanked for helping improve
a previous version of this paper. The authors acknowledge the assistance of
the editorial staff of Biogeosciences. Edited by: Emilio Marañón Reviewed by:
two anonymous referees
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