Gema Martínez-Méndez

Deep‐water masses are formed in the North‐Atlantic, making studies of δ ¹³ C of dissolved CO ₂ in this region key to monitor and understand the spreading of anthropogenic carbon into the ocean interior. The objective of this study is to quantify the Suess effect (SE) in North Atlantic water masses using sulfur hexafluoride (SF ₆ ) as the anthropogenic marker. We investigated water samples on two sections across the North‐Atlantic Ocean (∼48°N) from 2017/2018. Our data show that only bottom‐waters on the eastern side of the Mid‐Atlantic Ridge do not yet have a significant SE signal using SF ₆ . On the western side, a massive eddy visible in 2018 advected Labrador Sea Water labeled with a strong SE down to 2,000 m. These findings contribute to our understanding of the anthropogenic carbon signal in the deep ocean and highlight the importance to consider the SE in core top calibrations and paleoceanographic reconstructions using foraminiferal δ ¹³ C.

Palaeoceanographic proxies are essential in reconstructing past climate change and accurate estimates of past conditions require reliable calibrations. The chemical signature of benthic foraminiferal calcite reflects bottom water conditions, but the effects of their microhabitat, biomineralization, metabolism, ontogeny can challenge a straightforward interpretation of past seawater conditions. In this study we compared stable isotopes of oxygen (δ18OBF) and carbon (δ13CBF) values of age-constrained benthic foraminifera from surface sediments of the Southeast Pacific (SEP) with contemporary in-situ water column values for δ18O equilibrium calcite (δ18Oeqcal) and δ13C of dissolved inorganic carbon (δ13CDIC). The ages of benthic foraminifera, based on radiocarbon and oxygen isotope stratigraphy, show a cluster of older surface sediments sites at intermediate water depths off north Chile, likely associated with sediment erosion. Epifaunal benthic foraminifera δ18OBF values reveal a 1:1 relationship with δ18Oeqcal of ambient bottom waters across the region, while infaunal foraminifera show positive δ18OBF-δ18Oeqcal offsets. Epifaunal benthic foraminifera δ13CBF values closely match ambient bottom waters δ13CDIC in most cases. In contrast, infaunal benthic foraminifera show negative δ13CBF-δ13CDIC offset. At sites shallower than 1000 m, both epifaunal and infaunal benthic foraminifera display significant positive δ13CBF-δ13CDIC offsets. This offset is likely attributed to a combination of microbiological processes occurring within the bottom and pore waters under low to intermediate oxygen conditions, influencing the carbonate pool and the inorganic carbon speciation in the SEP. This is not likely a regionally restricted phenomenon and may have broader implications for improving the reliability of δ13C- and δ18O-based proxies in palaeoceanographic studies.

In this study, we used stable isotopes of oxygen (δ¹⁸O), deuterium (δD), and dissolved inorganic carbon (δ¹³C_DIC) in combination with temperature, salinity, oxygen, and nutrient concentrations to characterize the coastal (71°–78°W) and an oceanic (82°–98°W) water masses (SAAW—Subantarctic Surface Water; STW—Subtropical Water; ESSW—Equatorial Subsurface water; AAIW—Antarctic Intermediate Water; PDW—Pacific Deep Water) of the Southeast Pacific (SEP). The results show that δ¹⁸O and δD can be used to differentiate between SAAW‐STW, SAAW‐ESSW, and ESSW‐AAIW. δ¹³C_DIC signatures can be used to differentiate between STW‐ESSW (oceanic section), SAAW‐ESSW, ESSW‐AAIW, and AAIW‐PDW. Compared with the oceanic section, our new coastal section highlights differences in both the chemistry and geometry of water masses above 1,000 m. Previous paleoceanographic studies using marine sediments from the SEP continental margin used the present‐day hydrological oceanic transect to compare against, as the coastal section was not sufficiently characterized. We suggest that our new results of the coastal section should be used for past characterizations of the SEP water masses that are usually based on continental margin sediment samples.

Coeval changes in atmospheric CO₂ and ¹⁴C contents during the last deglaciation are often attributed to ocean circulation changes that released carbon stored in the deep ocean during the Last Glacial Maximum (LGM). Work is being done to generate records that allow for the identification of the exact mechanisms leading to the accumulation and release of carbon from the oceanic reservoir, but these mechanisms are still the subject of debate. Here we present foraminifera ¹⁴C data from five cores in a transect across the Chilean continental margin between ~540 and ~3,100 m depth spanning the last 20,000 years. Our data reveal that during the LGM, waters at ~2,000 m were 50% to 80% more depleted in Δ¹⁴C than waters at ~1,500 m when compared to modern values, consistent with the hypothesis of a glacial deep ocean carbon reservoir that was isolated from the atmosphere. During the deglaciation, our intermediate water records reveal homogenization in the Δ¹⁴C values between ~800 and ~1,500 m from ~16.5–14.5 ka cal BP to ~14–12 ka cal BP, which we interpret as deeper penetration of Antarctic Intermediate Water. While many questions still remain, this process could aid the ventilation of the deep ocean at the beginning of the deglaciation, contributing to the observed ~40 ppm rise in atmospheric pCO₂.

The southern westerly wind belt (SWW) interacts with the Antarctic Circumpolar Current and strongly impacts the Southern Ocean carbon budget, and Antarctic ice-sheet dynamics across glacial–interglacial cycles. We investigated precipitation-driven sediment input changes to the Southeast Pacific off the southern margin of the Atacama Desert over the past one million years, revealing strong precession (19/23-ka) cycles. Our simulations with 2 ocean–atmosphere general circulation models suggest that observed cyclic rainfall changes are linked to meridional shifts in water vapor transport from the tropical Pacific toward the southern Atacama Desert. These changes reflect a precessional modulation of the split in the austral winter South Pacific jet stream. For precession maxima, we infer significantly enhanced rainfall in the southern Atacama Desert due to a stronger South Pacific split jet with enhanced subtropical/subpolar jets, and a weaker midlatitude jet. Conversely, we derive dry conditions in northern Chile related to reduced subtropical/subpolar jets and an enhanced midlatitude jet for precession minima. The presence of precessional cycles in the Pacific SWW, and lack thereof in other basins, indicate that orbital-scale changes of the SWW were not zonally homogeneous across the Southern Hemisphere, in contrast to the hemispherewide shifts of the SWW suggested for glacial terminations. The strengthening of the jet is unique to the South Pacific realm and might have affected winter-controlled changes in the mixed layer depth, the formation of intermediate water, and the buildup of sea-ice around Antarctica, with implications for the global overturning circulation and the oceanic storage of atmospheric CO ₂ .

Rapid changes in ocean circulation and climate have been observed in marine-sediment and ice cores over the last glacial period and deglaciation, highlighting the non-linear character of the climate system and underlining the possibility of rapid climate shifts in response to anthropogenic greenhouse gas forcing. To date, these rapid changes in climate and ocean circulation are still not fully explained. One obstacle hindering progress in our understanding of the interactions between past ocean circulation and climate changes is the difficulty of accurately dating marine cores. Here, we present a set of 92 marine sediment cores from the Atlantic Ocean for which we have established age-depth models that are consistent with the Greenland GICC05 ice core chronology, and computed the associated dating uncertainties, using a new deposition modeling technique. This is the first set of consistently dated marine sediment cores enabling paleoclimate scientists to evaluate leads/lags between circulation and climate changes over vast regions of the Atlantic Ocean. Moreover, this data set is of direct use in paleoclimate modeling studies.

Subsurface waters from both hemispheres converge in the Western Equatorial Pacific (WEP), some of which form the Equatorial Undercurrent (EUC) that influences equatorial Pacific productivity across the basin. Measurements of nitrogen (N) and oxygen (O) isotope ratios in nitrate (δ¹⁵N_NO3 and δ¹⁸O_NO3), the isotope ratios of dissolved inorganic carbon (δ¹³C_DIC), and complementary biogeochemical tracers reveal that northern and southern WEP waters have distinct biogeochemical histories. Organic matter remineralization plays an important role in setting the nutrient characteristics on both sides of the WEP. However, remineralization in the northern WEP contributes a larger concentration of the nutrients, consistent with the older “age” of northern thermocline‐depth and intermediate‐depth waters. Remineralization introduces a relatively low δ¹⁵N_NO3 to northern waters, suggesting the production of sinking organic matter by N₂ fixation at the surface—consistent with the notion that N₂ fixation is quantitatively important in the North Pacific. In contrast, remineralization contributes elevated δ¹⁵N_NO3 to the southern WEP thermocline, which we hypothesize to derive from the vertical flux of high‐δ¹⁵N material at the southern edge of the equatorial upwelling. This signal potentially masks any imprint of N₂ fixation from South Pacific waters. The observations further suggest that the intrusion of high δ¹⁵N_NO3 and δ¹⁸O_NO3 waters from the eastern margins is more prominent in the northern than southern WEP. Together, these north‐south differences enable the examination of the hemispheric inputs to the EUC, which appear to derive predominantly from southern hemisphere waters.

Mg/Ca and stable oxygen isotope compositions (δ¹⁸O) of planktic foraminifera tests are commonly used as proxies to reconstruct past ocean conditions including variations in the vertical water column structure. Accurate proxy calibrations require thorough regional studies, since parameters such as calcification depth and temperature of planktic foraminifera depend on local environmental conditions. Here we present radiocarbon‐dated, modern surface sediment samples and water column data (temperature, salinity, and seawater δ¹⁸O) from the Western Pacific Warm Pool. Seawater δ¹⁸O (δ¹⁸O_SW) and salinity are used to calculate individual regressions for western Pacific surface and thermocline waters (δ¹⁸O_SW = 0.37 × S‐12.4 and δ¹⁸O_SW = 0.33 × S‐11.0). We combine shell δ¹⁸O and Mg/Ca with water column data to estimate calcification depths of several planktic foraminifera and establish regional Mg/Ca‐temperature calibrations. Globigerinoides ruber, Globigerinoides elongatus, and Globigerinoides sacculifer reflect mixed layer conditions. Pulleniatina obliquiloculata and Neogloboquadrina dutertrei and Globorotalia tumida preserve upper and lower thermocline conditions, respectively. Our multispecies Mg/Ca‐temperature calibration (Mg/Ca = 0.26exp0.097*T) matches published regressions. Assuming the same temperature sensitivity in all species, we propose species‐specific calibrations that can be used to reconstruct upper water column temperatures. The Mg/Ca temperature dependencies of G. ruber, G. elongatus, and G. tumida are similar to published equations. However, our data imply that calcification temperatures of G. sacculifer, P. obliquiloculata, and N. dutertrei are exceptionally warm in the western tropical Pacific and thus underestimated by previously published calibrations. Regional Mg/Ca‐temperature relations are best described by Mg/Ca = 0.24exp0.097*T for G. sacculifer and by Mg/Ca = 0.21exp0.097*T for P. obliquiloculata and N. dutertrei.