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  • 1
    Publication Date: 2017-08-16
    Description: Author Posting. © American Geophysical Union, 2017. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 32 (2017): 146–160, doi:10.1002/2016PA002976.
    Description: Coral skeletons are valuable archives of past ocean conditions. However, interpretation of coral paleotemperature records is confounded by uncertainties associated with single-element ratio thermometers, including Sr/Ca. A new approach, Sr-U, uses U/Ca to constrain the influence of Rayleigh fractionation on Sr/Ca. Here we build on the initial Pacific Porites Sr-U calibration to include multiple Atlantic and Pacific coral genera from multiple coral reef locations spanning a temperature range of 23.15–30.12°C. Accounting for the wintertime growth cessation of one Bermuda coral, we show that Sr-U is strongly correlated with the average water temperature at each location (r2 = 0.91, P 〈 0.001, n = 19). We applied the multispecies spatial calibration between Sr-U and temperature to reconstruct a 96 year long temperature record at Mona Island, Puerto Rico, using a coral not included in the calibration. Average Sr-U derived temperature for the period 1900–1996 is within 0.12°C of the average instrumental temperature at this site and captures the twentieth century warming trend of 0.06°C per decade. Sr-U also captures the timing of multiyear variability but with higher amplitude than implied by the instrumental data. Mean Sr-U temperatures and patterns of multiyear variability were replicated in a second coral in the same grid box. Conversely, Sr/Ca records from the same two corals were inconsistent with each other and failed to capture absolute sea temperatures, timing of multiyear variability, or the twentieth century warming trend. Our results suggest that coral Sr-U paleothermometry is a promising new tool for reconstruction of past ocean temperatures.
    Description: NSF Graduate Research Fellowships Grant Numbers: NSF-OCE-1338320, NSF-OCE-1031971, NSF-OCE-0926986; WHOI Access to the Sea Grant Numbers: 27500056, 0734826; NSF HRD; UPR Central Administration to EAHD through the Center for Applied Tropical Ecology and Conservation of UPR
    Description: 2017-08-16
    Keywords: Coral ; Temperature ; Paleoceangraphy ; Paleothermometry ; Global warming ; Biomineralization
    Repository Name: Woods Hole Open Access Server
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  • 2
    Publication Date: 2017-12-03
    Description: Author Posting. © American Geophysical Union, 2017. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 32 (2017): 512–530, doi:10.1002/2016PA003072.
    Description: The carbon isotope composition (δ13C) of seawater provides valuable insight on ocean circulation, air-sea exchange, the biological pump, and the global carbon cycle and is reflected by the δ13C of foraminifera tests. Here more than 1700 δ13C observations of the benthic foraminifera genus Cibicides from late Holocene sediments (δ13CCibnat) are compiled and compared with newly updated estimates of the natural (preindustrial) water column δ13C of dissolved inorganic carbon (δ13CDICnat) as part of the international Ocean Circulation and Carbon Cycling (OC3) project. Using selection criteria based on the spatial distance between samples, we find high correlation between δ13CCibnat and δ13CDICnat, confirming earlier work. Regression analyses indicate significant carbonate ion (−2.6 ± 0.4) × 10−3‰/(μmol kg−1) [CO32−] and pressure (−4.9 ± 1.7) × 10−5‰ m−1 (depth) effects, which we use to propose a new global calibration for predicting δ13CDICnat from δ13CCibnat. This calibration is shown to remove some systematic regional biases and decrease errors compared with the one-to-one relationship (δ13CDICnat = δ13CCibnat). However, these effects and the error reductions are relatively small, which suggests that most conclusions from previous studies using a one-to-one relationship remain robust. The remaining standard error of the regression is generally σ ≅ 0.25‰, with larger values found in the southeast Atlantic and Antarctic (σ ≅ 0.4‰) and for species other than Cibicides wuellerstorfi. Discussion of species effects and possible sources of the remaining errors may aid future attempts to improve the use of the benthic δ13C record.
    Description: U.S. National Science Foundation Grant Numbers: 1634719, 0926735, 1125181; Swiss National Science Foundation Grant Numbers: PP00P2_144811, 200021_163003; Canadian Institute for Advanced Research (CIFAR); Canadian Foundation for Innovation (CFI); Natural Sciences and Engineering Research Council (NSERC)
    Description: 2017-12-03
    Keywords: Carbon ; Isotopes ; Benthic ; Foraminifera ; Calibration
    Repository Name: Woods Hole Open Access Server
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  • 3
    Publication Date: 2018-11-01
    Description: © The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Paleoceanography and Paleoclimatology 33 (2018): 1013-1034, doi:10.1029/2018PA003408.
    Description: The chemical composition of benthic foraminifera from marine sediment cores provides information on how glacial subsurface water properties differed from modern, but separating the influence of changes in the origin and end‐member properties of subsurface water from changes in flows and mixing is challenging. Spatial gaps in coverage of glacial data add to the uncertainty. Here we present new data from cores collected from the Demerara Rise in the western tropical North Atlantic, including cores from the modern tropical phosphate maximum at Antarctic Intermediate Water (AAIW) depths. The results suggest lower phosphate concentration and higher carbonate saturation state within the phosphate maximum than modern despite similar carbon isotope values, consistent with less accumulation of respired nutrients and carbon, and reduced air‐sea gas exchange in source waters to the region. An inversion of new and published glacial data confirms these inferences and further suggests that lower preformed nutrients in AAIW, and partial replacement of this still relatively high‐nutrient AAIW with nutrient‐depleted, carbonate‐rich waters sourced from the region of the modern‐day northern subtropics, also contributed to the observed changes. The results suggest that glacial preformed and remineralized phosphate were lower throughout the upper Atlantic, but deep phosphate concentration was higher. The inversion, which relies on the fidelity of the paleoceanographic data, suggests that the partial replacement of North Atlantic sourced deep water by Southern Ocean Water was largely responsible for the apparent deep North Atlantic phosphate increase, rather than greater remineralization.
    Description: National Science Foundation (NSF) Grant Numbers: OCE‐0750880, OCE‐1335191, OCE‐1558341, OCE‐1536380; Woods Hole Oceanographic Institution (WHOI) Grant Numbers: 27007592, 27000808
    Keywords: Glacial Atlantic circulation ; Preformed phosphate ; Remineralized phosphate ; Antarctic Intermediate Water ; Nutrient redistribution ; Tropical phosphate maximum
    Repository Name: Woods Hole Open Access Server
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  • 4
    Publication Date: 2016-09-26
    Description: © The Author(s), 2015. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Paleoceanography 30 (2015): 353–368, doi:10.1002/2014PA002667.
    Description: Approximately synchronous with the onset of Heinrich Stadial 1 (HS1), δ13C decreased throughout most of the upper (~1000–2500 m) Atlantic, and at some deeper North Atlantic sites. This early deglacial δ13C decrease has been alternatively attributed to a reduced fraction of high-δ13C North Atlantic Deep Water (NADW) or to a decrease in the NADW δ13C source value. Here we present new benthic δ18O and δ13C records from three relatively shallow (~1450–1650 m) subpolar Northeast Atlantic cores. With published data from other cores, these data form a depth transect (~1200–3900 m) in the subpolar Northeast Atlantic. We compare Last Glacial Maximum (LGM) and HS1 data from this transect with data from a depth transect of cores from the Brazil Margin. The largest LGM-to-HS1 decreases in both benthic δ13C and δ18O occurred in upper waters containing the highest NADW fraction during the LGM. We show that the δ13C decrease can be explained entirely by a lower NADW δ13C source value, entirely by a decrease in the proportion of NADW relative to Southern Ocean Water, or by a combination of these mechanisms. However, building on insights from model simulations, we hypothesize that reduced ventilation due to a weakened but still active Atlantic Meridional Overturning Circulation also contributed to the low δ13C values in the upper North Atlantic. We suggest that the benthic δ18O gradients above ~2300 m at both core transects indicate the depth to which heat and North Atlantic deglacial freshwater had mixed into the subsurface ocean by early HS1.
    Description: The work was supported by NSF grants OCE13-35191, OCE07-50880, and OCE05-84911 to the Woods Hole Oceanographic Institution.
    Keywords: Heinrich Stadial 1 ; Deglacial d13C minimum ; Atlantic Circulation ; Benthic d18O ; Benthic d13C
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 5
    Publication Date: 2018-05-09
    Description: Author Posting. © American Geophysical Union, 2017. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 32 (2017): 1174–1194, doi:10.1002/2017PA003122.
    Description: 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 (δ18OSW) and salinity are used to calculate individual regressions for western Pacific surface and thermocline waters (δ18OSW = 0.37 × S-12.4 and δ18OSW = 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.
    Description: Bundesministerium für Bildung und Forschung (BMBF) Grant Number: 03G0228A; National Science Foundation (NSF) Grant Number: OCE1131371; DFG-Research Center/Cluster of Excellence “The Ocean in the Earth System”
    Description: 2018-05-09
    Keywords: Western Pacific Warm Pool ; Mg/Ca calibration ; Oxygen isotopes ; Planktic foraminifera ; Thermocline reconstruction
    Repository Name: Woods Hole Open Access Server
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  • 6
    Publication Date: 2016-08-06
    Description: Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 31 (2016): 252–265, doi:10.1002/2015PA002897.
    Description: Coral Sr/Ca is widely used to reconstruct past ocean temperatures. However, some studies report different Sr/Ca-temperature relationships for conspecifics on the same reef, with profound implications for interpretation of reconstructed temperatures. We assess whether these differences are attributable to small-scale oceanographic variability or “vital effects” associated with coral calcification and quantify the effect of intercolony differences on temperature estimates and uncertainties. Sr/Ca records from four massive Porites colonies growing on the east and west sides of Jarvis Island, central equatorial Pacific, were compared with in situ logger temperatures spanning 2002–2012. In general, Sr/Ca captured the occurrence of interannual sea surface temperature events but their amplitude was not consistently recorded by any of the corals. No long-term trend was identified in the instrumental data, yet Sr/Ca of one coral implied a statistically significant cooling trend while that of its neighbor implied a warming trend. Slopes of Sr/Ca-temperature regressions from the four different colonies were within error, but offsets in mean Sr/Ca rendered the regressions statistically distinct. Assuming that these relationships represent the full range of Sr/Ca-temperature calibrations in Jarvis Porites, we assessed how well Sr/Ca of a nonliving coral with an unknown Sr/Ca-temperature relationship can constrain past temperatures. Our results indicate that standard error of prediction methods underestimate the actual error as we could not reliably reconstruct the amplitude or frequency of El Niño–Southern Oscillation events as large as ± 2°C. Our results underscore the importance of characterizing the full range of temperature-Sr/Ca relationships at each study site to estimate true error.
    Description: This study was supported by an NSF Graduate Research Fellowship to A.A. and by NSF-OCE-0926986 and NSF-OCE-1031971.
    Description: 2016-08-06
    Keywords: Corals ; Paleoceanography ; Proxies
    Repository Name: Woods Hole Open Access Server
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  • 7
    Publication Date: 2017-04-05
    Description: Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 31 (2016): 1302–1314, doi:10.1002/2016PA002975.
    Description: Antarctic Intermediate Water is an essential limb of the Atlantic meridional overturning circulation that redistributes heat and nutrients within the Atlantic Ocean. Existing reconstructions have yielded conflicting results on the history of Antarctic Intermediate Water penetration into the Atlantic across the most recent glacial termination. In this study we present leachate, foraminiferal, and detrital neodymium isotope data from three intermediate-depth cores collected from the southern Brazil margin in the South Atlantic covering the past 25 kyr. These results reveal that strong chemical leaching following decarbonation does not extract past seawater neodymium composition in this location. The new foraminiferal records reveal no changes in seawater Nd isotopes during abrupt Northern Hemisphere cold events at these sites. We therefore conclude that there is no evidence for greater incursion of Antarctic Intermediate Water into the South Atlantic during either the Younger Dryas or Heinrich Stadial 1. We do, however, observe more radiogenic Nd isotope values in the intermediate-depth South Atlantic during the mid-Holocene. This radiogenic excursion coincides with evidence for a southward shift in the Southern Hemisphere westerlies that may have resulted in a greater entrainment of radiogenic Pacific-sourced water during intermediate water production in the Atlantic sector of the Southern Ocean. Our intermediate-depth records show similar values to a deglacial foraminiferal Nd isotope record from the deep South Atlantic during the Younger Dryas but are clearly distinct during the Last Glacial Maximum and Heinrich Stadial 1, demonstrating that the South Atlantic remained chemically stratified during Heinrich Stadial 1.
    Description: NERC Grant Numbers: NE/K005235/1, NE/F006047/1; NSF Grant Number: OCE -1335191; FAPESP Grant Number: 2012/17517-3; CAPES Grant Numbers: 1976/2014, 564/2015
    Description: 2017-04-05
    Keywords: Antarctic Intermediate Water ; Neodymium isotopes ; Degalciation ; South Atlantic
    Repository Name: Woods Hole Open Access Server
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  • 8
    Publication Date: 2018-04-24
    Description: Author Posting. © American Geophysical Union, 2017. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 32 (2017): 1036–1053, doi:10.1002/2017PA003092.
    Description: Antarctic Intermediate Water (AAIW) plays important roles in the global climate system and the global ocean nutrient and carbon cycles. However, it is unclear how AAIW responds to global climate changes. In particular, neodymium isotopic composition (εNd) reconstructions from different locations from the tropical Atlantic have led to a debate on the relationship between northward penetration of AAIW into the tropical Atlantic and the Atlantic meridional overturning circulation (AMOC) variability during the last deglaciation. We resolve this controversy by studying the transient oceanic evolution during the last deglaciation using a neodymium-enabled ocean model. Our results suggest a coherent response of AAIW and AMOC: when AMOC weakens, the northward penetration and transport of AAIW decrease while its depth and thickness increase. Our study highlights that as part of the return flow of the North Atlantic Deep Water, the northward penetration of AAIW in the Atlantic is determined predominately by AMOC intensity. Moreover, the inconsistency among different tropical Atlantic εNd reconstructions is reconciled by considering their corresponding core locations and depths, which were influenced by different water masses in the past. The very radiogenic water from the bottom of the Gulf of Mexico and the Caribbean Sea, which was previously overlooked in the interpretations of deglacial εNd variability, can be transported to shallow layers during active AMOC and modulates εNd in the tropical Atlantic. Changes in the AAIW core depth must also be considered. Thus, interpretation of εNd reconstructions from the tropical Atlantic is more complicated than suggested in previous studies.
    Description: NSF P2C2. Grant Numbers: NSF1401778, NSF1401802 DOE Grant Number: DE-SC0006744; NSFC Grant Numbers: 41630527, 41130105; Swiss National Science Foundation; WHOI Investing in Science Program; U.S. DOE the RGCM program; LDRD
    Description: 2018-04-24
    Keywords: AAIW ; AMOC ; Deglacial ; Neodymium isotope ; Paleocirculation tracer
    Repository Name: Woods Hole Open Access Server
    Type: Article
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