OS31C-01 INVITED
Modeling Nd Isotope Composition of Continental Inputs to the Ocean
Among the long-lived radiogenic isotope systems neodymium (Nd) is of particular interest to the oceanographic community because its residence time is shorter than the oceanic mixing time, causing regional differences in the Nd isotope composition of ocean basins and water masses (Piepgras et al., 1979). The unique Nd isotope compositions of water masses are created by inputs from the geologically diverse continents and then transported by the oceanic circulation. Old cratonic landmasses along passive margins impart a uniquely unradiogenic Nd isotope signature that reflects time-integrated low Sm/Nd values. Young active margins, in contrast, are characterized by radiogenic Nd isotope values that reflect the high Sm/Nd of the depleted mantle. This systematic can, in principle, be used to reconstruct ocean circulation and/or continental inputs to the ocean in the geologic past. Prerequisite for accurate reconstructions is a good understanding of changes in the geologic makeup of the continents and related isotopic signatures of the input fluxes to the ocean in the geologic past. Here we argue that the good negative correlation between the age of bedrock, as indicated by geologic maps, and the Nd isotope composition of suspended particles in rivers can be used to parameterize the Nd isotope composition of continental inputs to the ocean. We use a global assessment of bedrock ages based on the digital geologic map of the world at a scale of 1:25M in combination with more detailed digital bedrock maps for individual river drainage basins for which good isotope data are available (e.g., the Fraser, Mississippi, Hudson, Mackenzie rivers). Both the regional and global data define a linear array of epsilon Nd = -0.023 bedrock age (Myr) + 0.1. The lack of significant fractionation of Sm from Nd during erosion and weathering indicates that the particulate Nd isotope composition can also be used as a reliable proxy for the isotope composition of dissolved Nd in rivers. We use this correlation to estimate the isotope composition of runoff from 19 large-scale drainage regions into adjacent ocean basins (Graham et al., 1999). The results agree well with independent estimates of the Nd isotope composition of lithogenic inputs to the ocean (Jeandel et al., 2007). If glacial cover on cratonic shields surrounding the North Atlantic caused detrital and dissolved Nd fluxes to the ocean to decrease, significant changes in the isotope composition of the flux, and thus the Nd isotope composition of adjacent water masses, are to be expected. The amplitude of such glacial-interglacial variations may be modulated by continuing chemical exchange of continental detritus with seawater along continental margins. Reliable high-resolution records of the Nd isotope evolution of North Atlantic Deep Water (NADW) are needed to evaluate the effects of the Northern Hemisphere glaciation on the Nd isotope composition of NADW, and possible changes in its formation rate.
OS31C-02
Hafnium and Neodymium Isotopes in Atlantic Ocean Waters
Neodymium isotopic compositions (ICs) have been established as a tracer of water masses in the present and past oceans since the late 1970s. Hafnium isotopes share the capability of tracing water masses and in combination with Nd isotopes provide information on continental weathering regimes. Whereas Nd released during weathering reflects the bulk Nd IC of the weathered lithology, the released Hf is more radiogenic than the weathered lithology. This effect is due to highly variable Lu/Hf--ratios in rock--forming minerals ("zircon effect") and as a consequence physical weathering apparently leads to more congruent weathering of Hf than chemical weathering does. Our understanding of the Hf IC of seawater to date has been derived (with the exception of some as yet unpublished data from the Arctic and Pacific oceans (Zimmermann et al., in prep.)) from ferromanganese crusts and nodules, since Hf concentrations in seawater are low and have until recently hampered direct measurements of Hf IC of seawater. We present IC for the dissolved Hf and Nd in Atlantic seawater. Samples were taken mainly on a transect from the Bay of Biscay to Cape Town (RV Polarstern cruise ANT XXIII/1 in 2005). A few additional samples are from the Labrador Sea and the Drake Passage. Hafnium and Nd were pre--concentrated by iron co--precipitation from 60 to 140 liters of filtered (0.45 μm) seawater. Separation of Hf and Nd followed previously established ion chromatographic procedures. Hafnium and Nd ICs were measured by MC--ICPMS (Nu Plasma) with a 2σ external reproducibility of 0.65 and 0.3 \varepsilon--units, respectively. Sample sizes varied but were in most cases larger than 3ng of Hf. Surface seawater as well as deep water samples extending to ~5,000 m, plot on the "seawater array" defined previously from measurements of ferromanganese crusts and nodules. Surface seawater ICs are quite uniform for Hf ranging from \varepsilonHf = 0 to +2 at most sampling sites on the Atlantic transect. In the area of the Canary Islands, however, a strong shift with a maximum \varepsilonHf = +10.5 is observed. Neodymium ICs are more variable and can be divided into four distinct zones: the Bay of Biscay (\varepsilonNd ~ -10.9), the region of the Canary Islands (\varepsilonNd = -8.4), off North Africa (\varepsilonNd ~ -11.9) and the Angola and Cape Basins where \varepsilonNd values as low as -16.5 occur. Hafnium deep water ICs range from -2.1 in the Labrador Sea to +4.1 \varepsilonHf in the Drake Passage. Distinct ICs are observed for the Mediterranean Outflow Water (MOW, \varepsilonHf ~ +0.7) and the Antarctic Intermediate Water (AAIW, \varepsilonHf ~ +2.8). Deep and Bottom waters (North Atlantic Deep Water, NADW, Antarctic Bottom Water, AABW, and Circumpolar Deep Water, CDW) are quite uniform ranging from +3 to +4.2. In general, Hf and Nd ICs trace the deep water masses well. However, Nd ICs in the Angola Basin are shifted to unradiogenic signatures, requiring an additional external source of Nd to the seawater of this basin (Congo River, Namibian desert dust). Similarly, Hf ICs in North Atlantic Deep water close to the Mediterranean are shifted to less radiogenic isotopic compositions, probably reflecting contributions from MOW.
OS31C-03 INVITED
Radiogenic and stable isotope variations accompanying continental weathering
Many natural isotope systems, both stable and radiogenic, are sensitive to variations in weathering processes, but the difficulty remains in distinguishing variations that result from weathering from those caused by differences in rock type. One approach that circumvents this problem is the study of monolithologic catchments, where variations in physical and chemical weathering rates, runoff, catchment age, vegetative and glacial cover can be related to river chemistry. This study presents an overview of our recent work on radiogenic and stable isotopes in rivers and estuaries from Iceland, draining basaltic terrains, where variations in glacial cover result in a wide range of weathering conditions. Each radiogenic and stable isotope system reveals complementary information on the nature of the weathering process, and the estuarine data indicates how this signal is transferred to the oceans. For the dissolved riverine phase, in the absence of variations in rock type, the principal controls on isotope variations accompanying weathering are; (i) For many radiogenic isotope systems, preferential (incongruent) weathering of specific mineral phases, where those phases possess a markedly different parent/daughter ratio, and hence radiogenic isotope composition; (ii) For many stable isotope systems, preferential removal of an isotope into secondary phases formed during weathering, leaves residual waters depleted in that isotope. Despite the wide range of isotope compositions in the dissolved load, for Iceland it is the nature of weathering of the suspended load in the estuarine environment that likely dominates the signal to the oceans. Moreover, both the flux and nature of the suspended load are highly dependent on riverine discharge, and hence climate change. These results clearly demonstrate that weathering processes can exert a significant influence on the riverine isotope signal to the oceans, and for some isotopes marine sedimentary archives will preserve a record of changes in weathering in response to climatic or tectonic change. The challenge remains in deconvolving the effects of weathering from those caused by variations in rock type, or a simple change in the weathering flux.
OS31C-04
Strontium Isotopes, Basalt Weathering and Phanerozoic CO2
Using a combined model for the strontium and carbon cycles that distinguishes the weathering of volcanic and non-volcanic silicate rocks, the weathering of younger and older carbonates, basalt-seawater reaction, and marine carbonate burial, the ratio of volcanic (mainly basalt) weathering to total silicate weathering is calculated as a function of time from the oceanic record of 87Sr/86Sr. The volcanic proportion is then used to modify the equations for calculating atmospheric CO2 in the GEOCARBSULF model by the addition of a new non- dimensional volcanic weathering factor. The effect of uplift and physical erosion on weathering is also modified by using only the distribution over time of the abundance of sandstones and shales, and not Sr isotopic data that had been used previously. Results indicate large variations in the volcanic proportion of silicates undergoing weathering over time and uniformly lower CO2 values than GEOCARBSULF for the early Paleozoic and for the Mesozoic with the degree of lowering depending upon the 87Sr/86Sr of nonvolcanics undergoing weathering and the ratio of the intrinsic weatherability of volcanics to nonvolcanics. An increased minimum in CO2 during the Late Ordovician is in agreement with the presence of a continental glaciation at that time, and, using intrinsic volcanic/non-volcanic weatherability = 10, variations of Jurassic and Cretaceous CO2 agree with the independent work of Fletcher et al (2007) based on liverwort delta 13C values.
OS31C-05 INVITED
Using 232Th to monitor dissolved and total detrital inputs to the ocean
This study uses long lived thorium isotopes as a tracer for both total and dissolved detrital inputs to seawater over time. Th-232 in seawater is derived exclusively from detritus, and its presence in the dissolved phase results from partial dissolution of this material. 230Th is produced in situ at a predictable rate by the decay of uranium, and its subsequent removal by efficient adsorption onto settling particles provides a method to quantify both dissolved and total 232Th fluxes to the seafloor. Assuming a fixed Th-232 concentration in detritus allows calculation of detrital fluxes to the seafloor. Sediments were acid leached to extract adsorbed Th, and Th associated with carbonate phases. Select samples were also subject to total dissolution. Adsorbed (and total) core top ratios giving high dissolved (and total) Th-232 fluxes were measured in sediment cores from locations with high expected detrital inputs and vice versa. The absolute values of these fluxes are reasonable by comparison to global estimates of detrital inputs to the ocean. Down core results display higher 232Th/230Th ratios that are consistent with enhanced ice-age dust deposition in the central Atlantic, North Pacific and a Southern Ocean transect. In general the glacial increases indicated by both total and dissolved thorium are smaller than the order of magnitude changes recorded in high latitude ice cores. This new tracer may be used to evaluate a variety of scenarios regarding the ocean and atmosphere of the past, including dust fertilization schemes that link variations in paleo-productivity with changes in atmospheric carbon dioxide and global climate. The half-lives of both thorium isotopes are long enough to allow us to use our approach on sediments that span several glacial-interglacial cycles.
OS31C-06
The Atmospheric Supply of Terrestrial Authigenic Phosphate Minerals to Open Marine Sediments
Authigenic P-bearing minerals (Pauth), such as carbonate fluorapatite, form within shallow marine sediments as biological processes degrade organic matter and release associated phosphate to the dissolved pool during early diagenesis. Thus, Pauth is commonly used as a proxy for productivity in modern and ancient marine depositional environments. To help refine this proxy and further improve understanding of the marine P cycle, we investigated if dust deposition could supply terrestrially derived Pauth and other P-bearing phases to modern marine sediments. We used the SEDEX sequential extraction procedure to quantify the occurrence of P in ten samples of loess from the Chinese Loess Plateau, a major source of dust to the North Pacific Ocean (NPO). On average, 40% of the total P within Chinese Loess occurs as Pauth, 33% as detrital apatite (Pdet), 17% in organic matter (Porg), and 10% bound to Fe-Al oxides (Pox). Using eolian dust and total P accumulation rates reported for core LL44-GC3 taken from the central NPO, we find that ~86% of the total P accumulation within the central NPO could originate from the atmospheric deposition of Pauth and Pdet. Hence, productivity estimates based upon total P accumulation for this site are likely lower than previously estimated. Our findings suggest that marine productivity studies predicated on the measurement of Pauth need to quantify the fraction of Pauth supplied from terrestrial sources. This may be even more significant along continental margins where rivers can supply sediments with high concentrations of Pauth minerals.
OS31C-07
Sources of Fe to the Equatorial Pacific Ocean from the Holocene to Miocene
Biological productivity in the modern equatorial Pacific Ocean, a region with high nutrients and low chlorophyll, is currently limited by the micronutrient Fe. Whether this region was Fe limited in the past remains unclear. In order to test whether Fe was limiting in the past and to identify potential pathways of Fe delivery that could drive Fe fertilization (i.e., dust delivery from eolian inputs versus Fe supplied by the Equatorial Undercurrent), we chemically isolated the terrigenous material from marine sediment along a cross-equatorial meridional transect in the central equatorial Pacific at 140W and at Ocean Drilling Program Site 850 in the eastern equatorial Pacific. We quantified the contribution from each potential Fe-bearing terrigenous source using a complete suite of chemical- and isotopic discrimination strategies as well as multivariate statistical techniques. We find that the distribution of the terrigenous sources (i.e., Asian loess, South American ash, Papua New Guinea, and ocean island basalt) varies through time, latitude, and climate. Regardless of which method is used to determine accumulation rate, there also is no relationship between flux of any particular Fe source and climate. Moreover, there is no clear connection between a particular Fe source or pathway (eolian versus Undercurrent) to total productivity during the Last Glacial Maximum, Pleistocene glacial episodes, and the Miocene Biogenic Bloom. This would suggest an alternative process, such as an interoceanic reorganization of nutrient inventories, is responsible for past changes in carbon export in the open ocean, rather than simply Fe supply from dust and/or Equatorial Undercurrent processes.
OS31C-08
Atmospheric Supply of Trace Elements to the Oceans: Results from the CLIVAR Repeat Hydrography project 2003-2007
Aerosol and water column samples were collected from the A16N, P2, P16S/P16N, and I8S/I9N CLIVAR Repeat Hydrography cruises to study the impact of soluble trace element deposition on the chemistry of the upper water column (0-1000 meters). The "instantaneous" solubility of aerosol material was measured on replicate aerosol filters using ultrapure water and filtered surface seawater by passing 100 mL of the leach solution through the filters within 10 seconds. Rainfall samples were filtered ASAP using 0.4 um PCTE filters; an aliquot of unfiltered rain was also analyzed. Water column samples were collected using the CLIVAR trace metal clean rosette system loaded with 12 liter Teflon-coated GoFlo bottles. Seawater samples were filtered through 0.4 um PCTE filters and analyzed using either shipboard FIA analysis (for Al, Mn, Fe, and Fe(II)) or shore-based extraction and ICPMS analysis (for Al, Mn, Fe, Co, Ni, Cu, Zn, Cd, and Pb). The impact of recent atmospheric deposition on water column chemistry is most obvious in the North Atlantic where the Saharan dust plume deposits significant quantities of Al, Mn, Fe, and Co. Lower magnitude impact from the Asian dust plume is also observed in the North Pacific, and in the South Pacific near Tahiti from Australian desert dust. Dust loads in the South Pacific and southern Indian Ocean are much lower, and the impact of dust deposition on water column chemistry is not detectable south of 50S. Residence times for dissolved Al and Fe in the upper water column with respect to atmospheric deposition of soluble Al and Fe range from a few days in the North Atlantic to many months in the southern ocean.