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Research Interests: Geochemistry, Oceanography, Biology, Ecology, Cape Verde, and 14 moreACL, qPCR, North Atlantic, Trichodesmium, Aerosol, Iron, Pca, Anammox, BNF, GEOTRACES, nifH, Phosphate, Diazotrophs, and Phylotype
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Deep-sea corals are a promising new archive of paleoclimate. Coupled radiocarbon and U-series dates allow 14C to be used as a tracer of ocean circulation rate in the same manner as it is used in the modern ocean. Diagenetic alteration of... more
Deep-sea corals are a promising new archive of paleoclimate. Coupled radiocarbon and U-series dates allow 14C to be used as a tracer of ocean circulation rate in the same manner as it is used in the modern ocean. Diagenetic alteration of coral skeletons on the seafloor requires a thorough cleaning of contaminating phases of carbon. In addition, 10% of the coral must be chemically leached prior to dissolution to remove adsorbed modern CO2. A survey of modern samples from the full δ14C gradient in the deep ocean demonstrates that the coralline CaCO3 records the radiocarbon value of the dissolved inorganic carbon.
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Significance Low concentrations of the micronutrient iron in seawater are known to limit primary production and nitrogen fixation in large regions of the global ocean. Thus, it is important to constrain the sources and sinks controlling... more
Significance Low concentrations of the micronutrient iron in seawater are known to limit primary production and nitrogen fixation in large regions of the global ocean. Thus, it is important to constrain the sources and sinks controlling the marine dissolved iron distribution and consequent micronutrient supply to surface plankton. Although the major dissolved iron sources have been historically thought to be atmospheric dust inputs and fluxes from the continental margin, we show here the first data to our knowledge demonstrating that dissolved iron from hydrothermal vents can be transported thousands of kilometers from the venting site, which to date has only been suggested and modeled. Thus, hydrothermal vents must be considered when determining the marine dissolved iron inventory, especially in the abyssal ocean.
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Since the mid-1980s, our understanding of nutrient limitation of oceanic primary production has radically changed. Mesoscale iron addition experiments (FeAXs) have unequivocally shown that iron supply limits production in one-third of the... more
Since the mid-1980s, our understanding of nutrient limitation of oceanic primary production has radically changed. Mesoscale iron addition experiments (FeAXs) have unequivocally shown that iron supply limits production in one-third of the world ocean, where surface macronutrient concentrations are perennially high. The findings of these 12 FeAXs also reveal that iron supply exerts controls on the dynamics of plankton blooms, which in turn affect the biogeochemical cycles of carbon, nitrogen, silicon, and sulfur and ultimately influence the Earth climate system. However, extrapolation of the key results of FeAXs to regional and seasonal scales in some cases is limited because of differing modes of iron supply in FeAXs and in the modern and paleo-oceans. New research directions include quantification of the coupling of oceanic iron and carbon biogeochemistry.
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Various papers have been published during the past decade concerning Last Glacial Maximum (LGM) North Atlantic Deep Water (NADW) flow. Using somewhat different methods, they have produced somewhat contradictory results. This review... more
Various papers have been published during the past decade concerning Last Glacial Maximum (LGM) North Atlantic Deep Water (NADW) flow. Using somewhat different methods, they have produced somewhat contradictory results. This review considers both apparent and real conflicts concerning the data and their interpretation, and attempts to resolve them. Despite the earlier (contradictory) interpretations, currently there is a widespread belief that nutrient concentrations in deep cores from the North Atlantic increased during glacial times and that concentrations in the upper-deep and intermediate waters decreased at least slightly. It is also clear that further north in the basin (particularly at upper-deep and intermediate depths), nutrient concentrations were as low or perhaps even lower than those seen today. Data from the Caribbean Sea, ventilated by intermediate waters through an approximately 1800 m sill, indicate that lower nutrient levels were also found at intermediate depths i...
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Stable isotope, trace metal, alkenone paleothermometry, and radiocarbon methods have been applied to sediment cores in the western subpolar North Atlantic between Hudson Strait and Cape Hatteras to reveal the history of climate in that... more
Stable isotope, trace metal, alkenone paleothermometry, and radiocarbon methods have been applied to sediment cores in the western subpolar North Atlantic between Hudson Strait and Cape Hatteras to reveal the history of climate in that region over the past ∼11 kyr. We focus on cores from the Laurentian Fan, which is known to have rapid and continuous accumulation of hemipelagic sediment. Although results among our various proxy data are not always in agreement, the weight of the evidence (alkenone sea surface temperature (SST), δ18O and abundance of Globigerinoides ruber) indicates a continual cooling of surface waters over Laurentian Fan, from about 18°C in the early Holocene to about 8°C today. Alternatively, Mg/Ca data on planktonic foraminifera indicate no systematic change in Holocene SST. The inferred long‐term decrease in SST was probably driven by decreasing seasonality of Northern Hemisphere insolation. Two series of proxy data show the gradual cooling was interrupted by a ...
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A core‐top study of cadmium uptake into the shells of the aragonitic benthic foraminifera Hoeglundina elegans demonstrates that the Hoeglundina Cd/Ca partition coefficient is close to 1.0 throughout the ocean. Cd uptake by Hoeglundina is... more
A core‐top study of cadmium uptake into the shells of the aragonitic benthic foraminifera Hoeglundina elegans demonstrates that the Hoeglundina Cd/Ca partition coefficient is close to 1.0 throughout the ocean. Cd uptake by Hoeglundina is far less depth dependent than that of calcitic benthic foraminifera. Furthermore, manganese carbonate does not precipitate on these aragonitic shells, allowing for the recovery of Cd estimates in some samples where calcitic species are spoiled by contaminating overgrowths. Because Cd incorporation into Hoeglundina shows little depth dependence, a comparison of calcitic and Hoeglundina Cd data can be used to verify the assumption that the depth dependence observed for calcitic benthic foraminifera is time invariant. This comparison has been undertaken in downcore and last glacial maximum (LGM) samples from the northern Indian Ocean. Aragonitic and calcitic foraminiferal estimates for Cd in the LGM ocean are in excellent agreement. This result indicat...
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High‐resolution paleogeochemical data from the North Atlantic Ocean indicate that in the interval 15,000 to 10,000 14C years before present (B.P.) North Atlantic Deep Water (NADW) production was decreased or eliminated four times: at... more
High‐resolution paleogeochemical data from the North Atlantic Ocean indicate that in the interval 15,000 to 10,000 14C years before present (B.P.) North Atlantic Deep Water (NADW) production was decreased or eliminated four times: at about 14,500 (and probably older), 13,500, 12,000 and 10,500 years B.P. Each of these changes occurred at the same time as abrupt events of meltwater discharge to the surface ocean (inferred from oxygen isotope studies of planktonic foraminifera and from glacial geological studies on land). In addition, each of these times may be associated with brief episodes of cooler climate in the North Atlantic region, the best example of which is the Younger Dryas cooling of 10,500 years ago. These results support models linking meltwater discharge, decreased NADW production, and decreased North Atlantic heat flux.
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Research Interests: Biochemistry, Geology, Geochemistry, Chemistry, Carbon, and 15 moreModeling, Environmental Chemistry, Phosphorus, Atmospheric sciences, Surface Water, Scavenging, GEOTRACES, Global Biogeochemical Cycles, Trace element, Phosphate, Water Masses, Ligands, Ocean Currents, nutrient cycle, and Biogeochemical Cycle
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The oceanic distribution of cadmium resembles that of phosphorus. Because the cadmium content of foraminiferal shells is governed by the cadmium content of seawater, planktonic and benthic fossil shells can be used to infer nutrient... more
The oceanic distribution of cadmium resembles that of phosphorus. Because the cadmium content of foraminiferal shells is governed by the cadmium content of seawater, planktonic and benthic fossil shells can be used to infer nutrient distributions within ancient oceans. Empirical studies demonstrate that cadmium in benthic foraminiferal shells is related to the bottom water composition through a proportionality constant D ≃ 2.9. This constant is the same for each of the species studied: Cibicidoides wuellerstorfi, Cibicidoides kullenbergi, Nuttallides umbonifera, and Uvigerina.spp. Downcore cadmium data from high‐quality Pacific and Atlantic sediment cores suggest that the cadmium inventory of the ocean did not change significantly between the most recent glacial maximum and the present. Hence changes in the cadmium content of fossils at a site directly reflect changes in nutrient distributions due to altered oceanic circulation patterns. Studies of cadmium in Pleistocene sediments s...
Research Interests: Geology, Geochemistry, Oceanography, Paleoceanography, Ecology, and 6 moreQuaternary, Holocene, Seawater, Cadmium, Interglacial, and Tracer
Coupled radiocarbon and thorium-230 dates from benthic coral species reveal that the ventilation rate of the North Atlantic upper deep water varied greatly during the last deglaciation. Radiocarbon ages in several corals of the same age,... more
Coupled radiocarbon and thorium-230 dates from benthic coral species reveal that the ventilation rate of the North Atlantic upper deep water varied greatly during the last deglaciation. Radiocarbon ages in several corals of the same age, 15.41 ± 0.17 thousand years, and nearly the same depth, 1800 meters, in the western North Atlantic Ocean increased by as much as 670 years during the 30- to 160-year life spans of the samples. Cadmium/calcium ratios in one coral imply that the nutrient content of these deep waters also increased. Our data show that the deep ocean changed on decadal-centennial time scales during rapid changes in the surface ocean and the atmosphere.
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Climate over the past million years has been dominated by glaciation cycles with periods near 23,000, 41,000, and 100,000 years. In a linear version of the Milankovitch theory, the two shorter cycles can be explained as responses to... more
Climate over the past million years has been dominated by glaciation cycles with periods near 23,000, 41,000, and 100,000 years. In a linear version of the Milankovitch theory, the two shorter cycles can be explained as responses to insolation cycles driven by precession and obliquity. But the 100,000‐year radiation cycle (arising from eccentricity variation) is much too small in amplitude and too late in phase to produce the corresponding climate cycle by direct forcing. We present phase observations showing that the geographic progression of local responses over the 100,000‐year cycle is similar to the progression in the other two cycles, implying that a similar set of internal climatic mechanisms operates in all three. But the phase sequence in the 100,000‐year cycle requires a source of climatic inertia having a time constant (∼15,000 years) much larger than the other cycles (∼5,000 years). Our conceptual model identifies massive northern hemisphere ice sheets as this larger ine...