Gregory E. Ravizza

Biogenic barium fluxes to the deep sea: Implications for paleoproductivity reconstruction

Dymond et al. (1992) have recently proposed an algorithm to reconstruct paleoproductivity from biogenic Ba (bio-Ba) accumulation rates in sediments. Their equation is based on sediment trap data which indicate that Corg/bio-Ba ratios in settling particles are higher in the western Atlantic compared to the Pacific. From this observation they have suggested that the flux of bio-Ba to the seafloor may depend on dissolved Ba concentrations in intermediate and deep waters which are significantly higher in the Pacific compared to the Atlantic. Accordingly, they have introduced a factor related to dissolved Ba concentration in their equation as a variable which strongly influences paleoproductivity estimates. In an attempt to confirm the proposed dependency of bio-Ba fluxes to the seafloor on dissolved Ba concentrations in seawater we have compiled additional data on organic carbon and bio-Ba fluxes in the deep sea. These data confirm Dymond et al.s findings that settling particles have significantly higher Corg/bio-Ba in the western Atlantic compared to the Pacific. However, we also found lower ratios in traps deployed in the North Atlantic, similar to those found in the Pacific, while in the Panama Basin we found ratios as high as those in the western Atlantic. From these observations we conclude that dissolved Ba concentration is not an important factor in regulating the flux of bio-Ba to the seafloor. Instead, we propose that high Corg/bio-Ba ratios found in the western Atlantic, the Panama Basin, the Arabian Sea, and some stations in the Nordic Seas result from the addition of refractory organic carbon from nearby continents, shelves, or slopes. If that is confirmed, the algorithm proposed by Dymond et al. (1992) could be simplified and could provide a powerful means to estimate paleoproductivity. In addition, deviations from the Corg/bio-Ba ratios in settling particles could be used to estimate the input of continental or shelf-derived refractory organic matter into the deep sea.

The geochemical cycle of rhenium: a reconnaissance

Rhenium (Re) is one of a suite of elements (including uranium and molybdenum) that display conservative behavior in seawater and are enriched in anoxic sediments. The decay of187Re to187Os provides a geochronometer in ancient sedimentary rocks and gives rise to Os-isotopic variations in nature. In order to better characterize its sources to seawater, Re was measured in three major rivers (Amazon, Orinoco, Ganges-Brahmaputra) and some of their tributaries. Re concentrations span four orders of magnitude (from < 0.02 to 400 pmol/kg), with the highest concentrations found in rivers draining black shales in the Venezuelan Andes. Mainstream Re levels in the three rivers are between 1 and 10 pmol/kg, with a flux weighted average of 2.3 pmol/kg. The residence time for Re in the oceans is estimated to be 750,000 yr with respect to river inputs. Re profiles from the Atlantic and Pacific Oceans confirm that Re behaves conservatively in seawater, with no significant uptake onto particles and/or recycling within the water column. This is also true in the anoxic water column of the Black Sea. Re removal into anoxic sediments occurs at or below the sediment water interface, as demonstrated in sediment pore waters from Chesapeake Bay. In oxic sediments, Re is not cycled with manganese oxides, and it is not enriched in very slowly accumulating pelagic sediments with a large hydrogenous iron and manganese component, or in manganese nodules. Burial of Re in anoxic sediments, which accumulate on 0.3% of the ocean floor, removes approximately 50% of the riverine Re flux to the oceans. Hence, oceanic Re concentrations may be very sensitive to changes in the area of anoxic sedimentation.

The marine187Os/186Os record of the past 80 million years

We report new187Os/186Os data and Re and Os concentrations in metalliferous sediments from the Pacific to construct a composite Os isotope seawater evolution curve over the past 80 m.y. Analyses of four samples of upper Cretaceous age yield187Os/186Os values of between 3 and 6.5 and187Re/186Os values below 55. Mass balance calculations indicate that the pronounced minimum of about 2 in the Os isotope ratio of seawater at the K-T boundary probably reflects the enormous input of cosmogenic material into the oceans by the K-T impactor(s). Following a rapid recovery to187Os/186Os of 3.5 at 63 Ma, data for the early and middle part of the Cenozoic show an increase in187Os/186Os to about 6 at 15 Ma. Variations in the isotopic composition of leachable Os from slowly accumulating metalliferous sediments show large fluctuations over short time spans. In contrast, analyses of rapidly accumulating metalliferous carbonates do not exhibit the large oscillations observed in the pelagic clay leach data. These results together with sediment leaching experiments indicate that dissolution of non-hydrogenous Os can occur during the hydrogen peroxide leach and demonstrate that Os data from pelagic clay leachates do not always reflect the Os isotopic composition of seawater. New data for the late Cenozoic further substantiate the rapid increase in the187Os/186Os of seawater during the past 15 Ma. We interpret the correlation between the marine Sr and Os isotope records during this time period as evidence that weathering within the drainage basin of the Ganges-Brahmaputra river system is responsible for driving seawater Sr and Os toward more radiogenic isotopic compositions. The positive correlation between87Sr/86Sr and U concentration, the covariation of U and Re concentrations, and the high dissolved Re, U and Sr concentrations found in the Ganges-Brahmaputra river waters supports this interpretation. Accelerating uplift of many orogens worldwide over the past 15 Ma, especially during the last 5 Ma, could have contributed to the rapid increase in187Os/186Os from 6 to 8.5 over the past 15 Ma. Prior to 15 Ma the marine Sr and Os record are not tightly coupled. The heterogeneous distribution of different lithologies within eroding terrains may play an important role in decoupling the supplies of radiogenic Os and Sr to the oceans and account for the periods of decoupling of the marine Sr and Os isotope records.

The record of sea water 187Os/186Os variation through the Cenozoic

The variation of sea water 187Os/186Os during the Cenozoic has been determined by analyzing the hydrogenous Os released by acidic hydrogen peroxide leaching of layers in an abyssal North Pacific pelagic clay core. Overall, sea water 187Os/186Os has increased from 3.2 at 58 Ma to the present-day value of ∼ 8.6. This pattern is similar to the well-known 87Sr/86Sr evolution of sea water. The increase in the proportion of continental Os (187Os/186Os⩾ 10) could be due to enhanced weathering rates of black shales associated with the uplift of the Himalayas. Other elements associated with black shale weathering, such as U and P, should show effects in the sedimentary record.

Variations of the187Os/186Os ratio of seawater over the past 28 million years as inferred from metalliferous carbonates

Abstract The Os concentration and isotopic composition of metalliferous carbonates deposited on the East Pacific Rise over the past 28 Ma are reported with complimentary Sr isotope data. Variations in the Os isotopic composition of these samples are interpreted as a record of past changes in the Os isotopic composition of seawater. These results are consistent with isotopic analyses of leachable Os in pelagic clays which have also been interpreted as a record of the187Os/186Os ratio of seawater through time [1]. The metalliferous carbonate record clearly shows that seawater Os and Sr isotope systems are partially decoupled from one another over the past 28 Ma. Accelerated weathering of ancient organic-rich sediments is suggested as a possible mechanism to account for this decoupling and the rapid increase in the187Os/186Os ratio of seawater over the past 15 Ma. This rapid increase suggests that the seawater Os record can potentially be used as a stratigraphic tool in some Neogene marine deposits.

Os isotopes as tracers in seafloor hydrothermal systems: metalliferous deposits from the TAG hydrothermal area, 26°N Mid-Atlantic Ridge

Abstract Os isotopic analyses of a suite of samples from the TAG hydrothermal area, 26°N on the Mid-Atlantic Ridge, reveal evidence of release of ocean crust-derived Os to seawater and removal of seawater-derived Os to the seafloor. 187 Os 186 Os ratios of sulfide samples from the active TAG mound vary between 8.7 and 1.3. The highest ratios are indistinguishable from the inferred 187 Os 186 Os ratio of seawater. The incorporation of seawater-derived Os in these samples is interpreted as evidence of both entrainment of seawater into hydrothermal fluids and the addition of seawater Os to samples during sulfide oxidation. Several sulfide samples from the active TAG mound have 187 Os 186 Os ratios lower than the inferred seawater value, with the lowest ratio approaching values expected for pristine oceanic crust. These low 187 Os 186 Os ratios indicate that Os derived from alteration of the oceanic crust is released to seawater in association with venting of high temperature hydrothermal fluids. Precipitates from low-temperature hydrothermal fluids, Fe- and Mn-oxide deposits from the east wall of the axial rift valley, also have low 187 Os 186 Os ratios (4.2–6.8) relative to seawater. These data suggest Os isotopes may be sensitive tracers of low-temperature hydrothermal circulation on the seafloor, and that supply of ocean crust-derived Os to seawater is not strictly limited to high temperature hydrothermal vents. The Os concentration and isotopic composition of metalliferous sediment samples from a push core collected 125 m from the base of the active TAG mound are also reported. In this core metalliferous sediment horizons derived from mass wasting of altered sulfides exhibit variable 187 Os 186 Os ratios (4.8–8.7), reflecting mixing of seawater-derived and ocean crust-derived Os. In contrast, sediments in which the metalliferous component is derived from fall-out of hydrothermal material from the neutrally buoyant hydrothermal plume have 187 Os 186 Os ratios which vary over a narrow range (8.6–8.7), and appear to accurately record the Os isotopic composition of seawater.

The marine 187 Os/ 188 Os record of the Eocene–Oligocene transition: the interplay of weathering and glaciation

Abstract Osmium (Os) isotope analyses of bulk sediments from the South Atlantic, Equatorial Pacific, and the Italian Apennines yield a well-dated and coherent pattern of 187Os/188Os variation from the late Eocene to the early Oligocene. The resulting composite record demonstrates the global character of two prominent features of the low-resolution LL44-GPC3 Os isotope record [Pegram and Turekian, Geochim. Cosmochim. Acta 63 (1999) 4053–4058]. These are: (1) a pronounced minimum in 187Os/188Os (0.22–0.27) in the late Eocene, between 34 and 34.5 Ma, and (2) a subsequent rapid increase in 187Os/188Os, to approximately 0.6 by 32 Ma. An ultramafic weathering event and an increased influx of extraterrestrial particles to the Earth are discussed as alternative explanations for the late Eocene 187Os/188Os minimum. Comparison of the 187Os/188Os to benthic foraminiferal oxygen isotope records demonstrates that the nearly three-fold increase in 187Os/188Os from the late Eocene minimum coincides with the growth and decay of the first large ice sheet of the Oligocene (Oi1 [Miller et al., J. Geophys. Res. 96 (1991) 6829–6848]). The fine structure of the Os isotope record indicates that enhanced release of radiogenic Os, unrelated to the recovery from late Eocene minimum, lagged the initiation of the Oi1 event by roughly 0.5 Myr. This record, in conjunction with weathering studies in modern glacial soils [Blum, in: W.F. Ruddiman (Ed.), Tectonic Uplift and Climate Change, Plenum Press, New York, 1997, pp. 259–288; Peucker-Ehrenbrink and Blum, Geochim. Cosmochim. Acta 62 (1998) 3193–3203], suggests that exposure of freshly eroded material during deglaciation following Oi1 enhanced chemical weathering rates, and may have contributed to ice sheet stabilization by drawing down atmospheric carbon dioxide. The improved temporal resolution and age control of the refined Eocene–Oligocene Os isotope record also makes it possible to illustrate the late Eocene Os isotope excursion as a tool for global correlation of marine sediments.

A possible link between the seawater osmium isotope record and weathering of ancient sedimentary organic matter

The Os isotopic composition of leachable Os from a North Pacific pelagic clay sequence (Pegram et al., 1992 ) and bulk sediment samples of metalliferous carbonates deposited near the East Pacific Rise (Ravizza, 1993) display similar patterns of temporal variation (Fig. 1 ). This similarity indicates that the composite record of t87Os/186Os ratio variations reflects changes in the Os isotopic composition of seawater. Temporal changes in the Sr isotopic composition of seawater record changes in the relative rates of continental weathering and oceanic crustal alteration. The Os isotopic composition of seawater should be influenced by similar processes. In both the Rb-Sr and Re-Os systems, the continental crust is characterized by large time-integrated parent/ daughter ratios, compared to the deep Earth, imparting relatively radiogenic Sr and Os signatures to continental material. However, the seawater Sr and Os records are decoupled from one another over the past 27 Ma. From 27 to 15 Ma 87Sr/86Sr increased rapidly while the 18VOs/186Os ratio of seawater remained nearly constant. At 15 Ma the ~87Os/tS6Os ratio of seawater began to increase rapidly but the rate

Osmium isotopes and silver as tracers of anthropogenic metals in sediments from Massachusetts and Cape Cod bays

High concentrations of osmium (Os) and silver (Ag) and low 187Os/186Os ratios in Boston sewage make these elements sensitive tracers of the influence of sewage on marine sediments in Massachusetts and Cape Cod bays. Pristine marine sediments have Ag concentrations more than 200 times lower than sewage sludge, Os concentrations 10–40 times lower, and 187OS/186Os ratios six times higher. Surface sediments from both Massachusetts and Cape Cod bays exhibit both high Ag concentrations and low 187OS/186Os ratios indicating the influence of sewage particles on marine sediments in this region extends some 70 km from the point of sewage release. In detail, the distribution of Os and Ag do not support a model of simple physical mixing of sewage particles with normal marine sediments. Deviations from the mixing model may be the result of fractionation of Os and Ag in the marine environment, and [or] independent temporal variations in the Os and Ag content of the waste stream. The results of this investigation suggest that osmium isotopes may be widely applicable as tracers of the influence of sewage on sediments in estuarine environments and that subtle variations in the isotopic composition of Os in the waste stream may help constrain the sources of Os and other metals delivered to the environment.

Mass failure of the North Atlantic margin triggered by the Cretaceous-Paleogene bolide impact

The 65 Ma Chicxulub impact is known to have triggered large submarine slope failures near the crater in the Gulf of Mexico and Caribbean. Here we show that the impact also precipitated massive submarine failure of the continental margin around the western North Atlantic. We have identified mass-flow deposits containing impact ejecta of Cretaceous-Paleogene boundary age in two deep-sea cores more than 700 km from the continental margin. The mass flows must have originated from the continental margin because they contain microfossils typical of shallow-water environments and sedimentary features indicative of deposition by gravity currents. Geophysical data suggest that the mass-wasting deposits may be present over the entire western North Atlantic from Puerto Rico to the Grand Banks of Canada, and that much of the eastern margin of North America catastrophically failed during the Cretaceous-Paleogene impact.