Changing atmospheric Δ^(14)C and the record of deep water paleoventilation ages

We propose a new calculation method to better estimate the deep water ventilation age from benthic-planktonic foraminifera ^(14)C ages. Our study is motivated by the fact that changes in atmospheric Δ^(14)C through time can cause contemporary benthic and planktonic foraminifera to have different initial Δ^(14)C values. This effect can cause spurious ventilation age changes to be interpreted from the geologic data. Using a new calculation method, ^(14)C projection ages, we recalculate the data from the Pacific Ocean. Contrary to previous results, we find that the Pacific intermediate and deep waters were about 600 years older than today at the last glacial maximum. In addition, there are possible signals of ventilation age change prior to ice sheet melting and at the Younger Dryas. However, the data are still too sparse to constrain these ventilation transients

Cool tropical temperatures shift the global δ18O-T relationship: An explanation for the ice core δ18O- borehole thermometry conflict?

The discrepancy between central Greenland borehole temperatures and the isotopic composition of Last Glacial Maximum ice can be explained by a shift in the δ[superscript 18]O-T relationship for the hydrological cycle linked to cooler tropical temperatures. This concept is illustrated using a simple Rayleigh distillation model. An estimate for α=Δδ[superscript 18] O/ΔT (LGM-Holocene) of −0.37 ‰/°C is determined with a simple graphical technique.National Science Foundation (U.S.) (Grant OCE9402198)United States. National Oceanic and Atmospheric Administration (Grant NA46GP0282

The marine geochemistry of trace metals

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution April, 1976The marine geochemical cycles of iron, copper, nickel, and cadmium were studied in order to provide a basis for oceanographic models for trace metals. Copper, nickel, and cadmium can be determined in a 100 ml seawater sample using cobalt pyrrolidine dithioacarbamate chelate coprecipitation and graphite atomizer atomic absorption spectrometry. Concentration ranges likely to be encountered and estimated (1δ) analytical precisions are copper, 1 to 6 nanomole/kg (±0.1); nickel, 3 to 12 nanomole/kg (±0.3); and cadmium, 0. 0 to 1.1 nanomole/kg (±0.1). The technique may be applied to freshwater samples with slight modification. A survey of several east coast U. S. estuaries established that an iron removal process occurs commonly when rivers mix with seawater. Laboratory mixing experiments using water from the Merrimack River (Mass.) and the Mullica River (New Jersey) demonstrated that rapid iron precipitation occurs as negatively-charged iron-organic colloids react with seawater cations and coagulate. This phenomenom was modeled using a synthetic, organic-stabilized colloidal suspension of goethite. The generality of the mechanism suggests that the world-average net river input of iron to the oceans is less than 1 μmole/kg of river water, an order of magnitude below previous estimates. Profiles of cadmium were obtained for 3 GEOSECS stations in the Pacific Ocean. Cadmium shows a consistent linear correlation with phosphate which demonstrates that cadmium is regenerated in a shallow cycle within the water column. The water column correlation is consistent with data on cadmium in marine organisms. Cadmium is enriched in upwelling regions which explains reports of cadmium enrichment in plankton from the Baja California upwelling region. Copper and nickel measurements have been made for three profiles from the Pacific Ocean. Observed copper concentrations range from 1 to 6 nanomole/kg; nickel varies from 3 to 12 nanomole/kg. Copper and nickel are removed from surface waters by uptake into organisms. As noted previously, nickel is regenerated partially in a shallow cycle (like P) and also in a deep cycle (like Ba). Copper is regenerated from biological debris at the bottom but is also scavenged from the mid and deep water column by an undetermined mechanism. The scavenging residence time is 1400 years. An estimate for the continental input of Ni, 7 nanomole/kg of river water, and Cu, 18 nanomole/kg of river water, was derived from measurements in the Amazon estuary. The oceanic residence times for nickel and copper are about 10,000 years. Evidence available on the uptake laws for trace metals by plankton suggests that a consistent relationship between the uptake law and the depth of regeneration may apply.Money in support of this research came at various times from the ONR, MIT UROP office, and a grant from the Doherty Foundation

Modeling the global ocean iron cycle

Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 18 (2004): GB1002, doi:10.1029/2003GB002061.We describe a model of the ocean transport and biogeochemical cycling of iron and the subsequent control on export production and macronutrient distributions. Ocean transport of phosphorus and iron are represented by a highly idealized six-box ocean model. Export production is parameterized simply; it is limited by light, phosphate, and iron availability in the surface ocean. We prescribe the regional variations in aeolian deposition of iron and examine three parameterizations of iron cycling in the deep ocean: (1) net scavenging onto particles, the simplest model; (2) scavenging and desorption of iron to and from particles, analogous to thorium; and (3) complexation. Provided that some unknown parameter values can be set appropriately, all three biogeochemical models are capable of reproducing the broad features of the iron distribution observed in the modern ocean and explicitly lead to regions of elevated surface phosphate, particularly in the Southern Ocean. We compare the sensitivity of Southern Ocean surface macronutrient concentration to increased aeolian dust supply for each parameterization. Both scavenging-based representations respond to increasing dust supply with a drawdown of surface phosphate in an almost linear relationship. The complexation parameterization, however, asymptotes toward a limited drawdown of phosphate under the assumption that ligand production does not respond to increased dust flux. In the scavenging based models, deep water iron concentrations and, therefore, upwelled iron continually increase with greater dust supply. In contrast, the availability of complexing ligand provides an upper limit for the deep water iron concentration in the latter model.M. J. F. is grateful for funding from NOAA (NA16GP2988) and NSSF (OCE-336839). P. P. is grateful to the MIT Martin Fellowship and NASA Earth System Science Fellowship (NGT5- 30362) for funding

Trivalent chromium isotopes in the eastern tropical North Pacific oxygen-deficient zone

Author Posting. © National Academy of Sciences, 2021. This article is posted here by permission of National Academy of Sciences for personal use, not for redistribution. The definitive version was published in Proceedings of the National Academy of Sciences of the United States of America 118(8), (2021): e1918605118, https://doi.org/10.1073/pnas.1918605118.Changes in chromium (Cr) isotope ratios due to fractionation between trivalent [Cr(III)] and hexavalent [Cr(VI)] are being utilized by geologists to infer oxygen conditions in past environments. However, there is little information available on Cr in the modern ocean to ground-truth these inferences. Transformations between the two chromium species are important processes in oceanic Cr cycling. Here we present profiles of hexavalent and trivalent Cr concentrations and stable isotope ratios from the eastern tropical North Pacific (ETNP) oxygen-deficient zone (ODZ) which support theoretical and experimental studies that predict that lighter Cr is preferentially reduced in low-oxygen environments and that residual dissolved Cr becomes heavier due to removal of particle-reactive Cr(III) on sinking particles. The Cr(III) maximum dominantly occurs in the upper portion of the ODZ, implying that microbial activity (dependent on the sinking flux of organic matter) may be the dominant mechanism for this transformation, rather than a simple inorganic chemical conversion between the species depending on the redox potential.We thank chief scientist Gabrielle Rocap for accommodating us on cruises Roger Revelle 1804-5 and Kilo Moana 19-20 (sponsored by NSF Grant DEB-1542240 to G. Rocap, A. Devol, R. Kiel, and C. Deutch), Jim Moffett for helping with sampling on these cruises, and Mark Altabet and Frank Stewart for collecting the samples from station 2T on cruise New Horizon 1410. This research was supported by NSF Grant OCE-1736996 (to E.A.B.) and by a fellowship from the Massachusetts Institute of Technology/Woods Hole Oceanographic Institution Joint Program in Oceanography

Distal transport of dissolved hydrothermal iron in the deep South Pacific Ocean

Until recently, hydrothermal vents were not considered to be an important source to the marine dissolved Fe (dFe) inventory because hydrothermal Fe was believed to precipitate quantitatively near the vent site. Based on recent abyssal dFe enrichments near hydrothermal vents, however, the leaky vent hypothesis [Toner BM, et al. (2012) Oceanography 25(1):209–212] argues that some hydrothermal Fe persists in the dissolved phase and contributes a significant flux of dFe to the global ocean. We show here the first, to our knowledge, dFe (<0.4 µm) measurements from the abyssal southeast and southwest Pacific Ocean, where dFe of 1.0–1.5 nmol/kg near 2,000 m depth (0.4–0.9 nmol/kg above typical deep-sea dFe concentrations) was determined to be hydrothermally derived based on its correlation with primordial [superscript 3]He and dissolved Mn (dFe:[superscript 3]He of 0.9–2.7 × 10[superscript 6]). Given the known sites of hydrothermal venting in these regions, this dFe must have been transported thousands of kilometers away from its vent site to reach our sampling stations. Additionally, changes in the size partitioning of the hydrothermal dFe between soluble (<0.02 µm) and colloidal (0.02–0.4 µm) phases with increasing distance from the vents indicate that dFe transformations continue to occur far from the vent source. This study confirms that although the southern East Pacific Rise only leaks 0.02–1% of total Fe vented into the abyssal Pacific, this dFe persists thousands of kilometers away from the vent source with sufficient magnitude that hydrothermal vents can have far-field effects on global dFe distributions and inventories (≥3% of global aerosol dFe input).National Science Foundation (U.S.). Graduate Research Fellowship (Award 0645960)Center for Microbial Oceanography: Research and Education (NSF-OIA Award EF-0424599)Gordon and Betty Moore Foundatio

The GEOTRACES Intermediate Data Product 2014

The GEOTRACES Intermediate Data Product 2014 (IDP2014) is the first publicly available data product of the international GEOTRACES programme, and contains data measured and quality controlled before the end of 2013. It consists of two parts: (1) a compilation of digital data for more than 200 trace elements and isotopes (TEIs) as well as classical hydrographic parameters, and (2) the eGEOTRACES Electronic Atlas providing a strongly inter-linked on-line atlas including more than 300 section plots and 90 animated 3D scenes. The IDP2014 covers the Atlantic, Arctic, and Indian oceans, exhibiting highest data density in the Atlantic. The TEI data in the IDP2014 are quality controlled by careful assessment of intercalibration results and multi-laboratory data comparisons at cross-over stations. The digital data are provided in several formats, including ASCII spreadsheet, Excel spreadsheet, netCDF, and Ocean Data View collection. In addition to the actual data values the IDP2014 also contains data quality flags and 1-σ data error values where available. Quality flags and error values are useful for data filtering. Metadata about data originators, analytical methods and original publications related to the data are linked to the data in an easily accessible way. The eGEOTRACES Electronic Atlas is the visual representation of the IDP2014 data providing section plots and a new kind of animated 3D scenes. The basin-wide 3D scenes allow for viewing of data from many cruises at the same time, thereby providing quick overviews of large-scale tracer distributions. In addition, the 3D scenes provide geographical and bathymetric context that is crucial for the interpretation and assessment of observed tracer plumes, as well as for making inferences about controlling processes.National Science Foundation (U.S.) (OCE-0608600)National Science Foundation (U.S.) (OCE-0938349)National Science Foundation (U.S.) (OCE-1243377

Variability of the North Atlantic thermohaline circulation during the last interglacial period

Studies of natural climate variability are essential for evaluating its future evolution. Greenland ice cores suggest that the modern warm period (the Holocene) has been relatively stable for the past 9,000 years. Much less is known about other warm interglacial periods, which comprise less than 10% of the climate record during the past 2.5 million years. Here we present high-resolution ocean sediment records of surface and deep-water variables from the Bermuda Rise spanning the last interglacial period, about 118,000–127,000 years ago. In general, deep-water chemical changes are coincident with transitions in surface climate at this site. The records do not show any substantial fluctuations relative to the much higher variability observed during the preceding and subsequent cool climates. The relatively stable interglacial period begins and ends with abrupt changes in deep-water flow. We estimate, using ^(230)Th measurements to constrain the chronology, that transitions occur in less than 400 years