American Geophysical Union (AGU)

We used sediment traps to define the particulate fluxes of barium and organic carbon and investigate the use of barium as a proxy for ocean fertility. Strong correlations between C_org and Ba fluxes indicate a link between upper ocean biological processes and barium flux to the seafloor. The ratio of organic carbon to barium decreases systematically with water depth. Data from 10 sites indicate that organic debris settling from the 200‐m depth has a C_org /Ba ratio of approximately 200. The systematic decrease in this ratio with increasing water depth results from the simultaneous decay of organic matter and uptake of Ba in settling particles. This behavior provides additional evidence that the formation of barite in oceanic particles is a consequence of decomposition/uptake in microenvironments rather than the secretion of barite by specific organisms. The decrease of the Corg/Ba ratio with depth is greatest in the North Pacific followed by the equatorial Pacific and is lowest in the western Atlantic. Since this spatial pattern is consistent with the variations in the deep‐ocean barium contents which increase along the path of bottom water flow from the Atlantic to the North Pacific, it suggests that the particulate barium uptake and flux is enhanced by higher barium contents in the intermediate and deep waters of the ocean. Consequently, we have combined our particle flux data with existing water column Ba data to define an algorithm relating new productivity, dissolved barium contents, water depth, and particulate barium flux. This relationship provides a basis of applying barium flux measurements in sediments to estimating new production. In order to use barium as an indicator of productivity, it will be necessary to evaluate inputs from hydrothermal and aluminosilicate sources and xenophyophors. The application of a sequential leach procedure to the trap material indicates that 50‐70% of the Ba in settling particles is in the form of barite and the remaining is adsorbed or bound to carbonates. Normative analysis demonstrates that in nearshore areas the contribution of barium from aluminosilicate sources can dominate that from biogenic inputs. It appears that normative estimates of biogenic barium contents can be made with accuracy if less than 50% of the Ba is associated with aluminosilicates; i.e., is of terrigenous origin. Since diagenetic mobilization of Ba can occur in reduced and suboxic sediments, highly productive nearshore areas also are likely to be inappropriate sites to use Ba measurements as productivity indicators. Comparisons between the rain rates of particulate Ba to the seafloor and the burial rate indicate that approximately 30% of the Ba rain is preserved. Although the preservation factor does not appear to be constant, it may be possible to predict the extent of preservation from an empirical relationship with the mass accumulation rate. These observations indicate that measurement of Ba burial fluxes in sediments can provide quantitative information on the paleoproductivity of the oceans. Joining the relationship between barium rain and burial with the barium and organic carbon algorithm, we make estimates of the new production in the northern California Current during the last 18,000 years. This calculation suggests that new production was at least a factor of 2 lower at this site during the last glacial maximum.

Sedimentary molybdenum, [Mo]_s, has been widely used as a proxy for benthic redox potential owing to its generally strong enrichment in organic‐rich marine facies deposited under oxygen‐depleted conditions. A detailed analysis of [Mo]_s–total organic carbon (TOC) covariation in modern anoxic marine environments and its relationship to ambient water chemistry suggests that (1) [Mo]_s, while useful in distinguishing oxic from anoxic facies, is not related in a simple manner to dissolved sulfide concentrations within euxinic environments and (2) patterns of [Mo]_s‐TOC covariation can provide information about paleohydrographic conditions, especially the degree of restriction of the subchemoclinal water mass and temporal changes thereof related to deepwater renewal. These inferences are based on data from four anoxic silled basins (the Black Sea, Framvaren Fjord, Cariaco Basin, and Saanich Inlet) and one upwelling zone (the Namibian Shelf), representing a spectrum of aqueous chemical conditions related to water mass restriction. In the silled‐basin environments, increasing restriction is correlated with a systematic decrease in [Mo]_s/TOC ratios, from ∼45 ± 5 for Saanich Inlet to ∼4.5 ± 1 for the Black Sea. This variation reflects control of [Mo]_s by [Mo]_aq, which becomes depleted in stagnant basins through removal to the sediment without adequate resupply by deepwater renewal (the “basin reservoir effect”). The temporal dynamics of this process are revealed by high‐resolution chemostratigraphic data from Framvaren Fjord and Cariaco Basin sediment cores, which exhibit long‐term trends toward lower [Mo]_s/TOC ratios following development of water column stratification and deepwater anoxia. Mo burial fluxes peak in weakly sulfidic environments such as Saanich Inlet (owing to a combination of greater [Mo]_aq availability and enhanced Mo transport to the sediment‐water interface via Fe‐Mn redox cycling) and are lower in strongly sulfidic environments such as the Black Sea and Framvaren Fjord. These observations demonstrate that, at timescales associated with deepwater renewal in anoxic silled basins, decreased sedimentary Mo concentrations and burial fluxes are associated with lower benthic redox potentials (i.e., more sulfidic conditions). These conclusions apply only to anoxic marine environments exhibiting some degree of water mass restriction (e.g., silled basins) and are not valid for low‐oxygen facies in open marine settings such as continent‐margin upwelling systems.

Paired Mg/Ca and δ¹⁸O measurements on planktonic foraminiferal species (G. ruber white, G. ruber pink, G. sacculifer, G. conglobatus, G. aequilateralis, O. universa, N. dutertrei, P. obliquiloculata, G. inflata, G. truncatulinoides, G. hirsuta, and G. crassaformis) from a 6‐year sediment trap time series in the Sargasso Sea were used to define the sensitivity of foraminiferal Mg/Ca to calcification temperature. Habitat depths and calcification temperatures were estimated from comparison of δ¹⁸O of foraminifera with equilibrium calcite, based on historical temperature and salinity data. When considered together, Mg/Ca (mmol/mol) of all species, except two, show a significant (r = 0.93) relationship with temperature (T °C) of the form Mg/Ca = 0.38 (±0.02) exp 0.090 (±0.003)T, equivalent to a 9.0 ± 0.3% change in Mg/Ca for a 1°C change in temperature. Small differences exist in calibrations between species and between different size fractions of the same species. O. universa and G. aequilateralis have higher Mg/Ca than other species, and in general, data can be best described with the same temperature sensitivity for all species and pre‐exponential constants in the sequence O. universa > G. aequilateralis ≈ G. bulloides > G. ruber ≈ G. sacculifer ≈ other species. This approach gives an accuracy of ±1.2°C in the estimation of calcification temperature. The ∼9% sensitivity to temperature is similar to published studies from culture and core top calibrations, but differences exist from some literature values of pre‐exponential constants. Different cleaning methodologies and artefacts of core top dissolution are probably implicated, and perhaps environmental factors yet understood. Planktonic foraminiferal Mg/Ca temperature estimates can be used for reconstructing surface temperatures and mixed and thermocline temperatures (using G. ruber pink, G. ruber white, G. sacculifer, N. dutertrei, P. obliquiloculata, etc.). The existence of a single Mg thermometry equation is valuable for extinct species, although use of species‐specific equations will, where statistically significant, provide more accurate evaluation of Mg/Ca paleotemperature.

Use of sedimentary organic carbon concentrations as a record of paleoceanographic conditions is complicated by an insufficient understanding of the mechanisms controlling present‐day variations in the organic matter content of surface open ocean sediments. This paper is a review of organic carbon distributions in marine sediments, the global marine balance of particulate and dissolved organic carbon and the processes controlling organic matter diagenesis. The discussion focuses on the last topic with the intention of bringing together mass balance and organic chemical evidence for mechanisms that control the preservation of organic matter in open ocean sediments.

We examine the relationships between ocean ventilation, primary production, water column anoxia, and benthic regeneration of phosphorus using a mass balance model of the coupled marine biogeochemical cycles of carbon (C) and phosphorus (P). The elemental cycles are coupled via the Redfield C/P ratio of marine phytoplankton and the C/P ratio of organic matter preserved in marine sediments. The model assumes that on geologic timescales, net primary production in the oceans is limited by the upwelling of dissolved phosphorus to the photic zone. The model incorporates the dependence on bottom water oxygenation of the regeneration of nutrient phosphorus from particulate matter deposited at the water‐sediment interface. Evidence from marine and lacustrine settings, modern and ancient, demonstrates that sedimentary burial of phosphorus associated with organic matter and ferric oxyhydroxides decreases when bottom water anoxia‐dysoxia expands. Steady state simulations show that a reduction in the rate of thermohaline circulation, or a decrease of the oxygen content of downwelling water masses, intensifies water column anoxia‐dysoxia and at the same time increases surface water productivity. The first effect reflects the declining supply of oxygen to the deeper parts of the ocean. The second effect is caused by the enhanced benthic regeneration of phosphorus from organic matter and ferric oxyhydroxides. Sedimentary burial of organic carbon and authigenic calcium phosphate mineral (francolite), on the other hand, is promoted by reduced ocean ventilation. According to the model, global‐scale anoxia‐dysoxia leads to a more efficient recycling of reactive phosphorus within the ocean system. Consequently, higher rates of primary production and organic carbon burial can be achieved, even when the continental supply of reactive phosphorus to the oceans remains unchanged.

Based on detailed reconstructions of global distribution patterns, both paleoproductivity and the benthic δ¹³C record of CO₂, which is dissolved in the deep ocean, strongly differed between the Last Glacial Maximum and the Holocene. With the onset of Termination I about 15,000 years ago, the new (export) production of low‐ and mid‐latitude upwelling cells started to decline by more than 2‐4 Gt carbon/year. This reduction is regarded as a main factor leading to both the simultaneous rise in atmospheric CO₂ as recorded in ice cores and, with a slight delay of more than 1000 years, to a large‐scale gradual CO₂ depletion of the deep ocean by about 650 Gt C. This estimate is based on an average increase in benthic δ¹³C by 0.4–0.5‰. The decrease in new production also matches a clear ¹³C depletion of organic matter, possibly recording an end of extreme nutrient utilization in upwelling cells. As shown by Sarnthein et al., [1987], the productivity reversal appears to be triggered by a rapid reduction in the strength of meridional trades, which in turn was linked via a shrinking extent of sea ice to a massive increase in high‐latitude insolation, i.e., to orbital forcing as primary cause.

Five distinct changes in the paleoenvironment of the Japan Sea within the last 85,000 years are revealed from the sedimentary record of a piston core recovered from the Oki Ridge. Changes in both surface and deepwater conditions are registered by changes in lithology, calcium carbonate content, organic carbon content, oxygen and carbon isotope ratios, and microfossil assemblages including calcareous nannoplankton, diatoms, radiolaria, and foraminifera. Between 85 and 27 ka the warm Tsushima Current did not flow into the Japan Sea, and cold surface water conditions prevailed. Environments at the seafloor fluctuated between dysaerobic to weakly oxic conditions. Between 27 and 20 ka, freshwater input to the Japan Sea, probably from the Huang Ho River in China, stratified the water column, and the severe anoxic conditions eliminated most benthic fauna. Between 20 and 10 ka the cold Oyashio Current flowed into the Japan Sea through the Tsugaru Strait, reestablishing deepwater ventilation. Shallow water benthic assemblages of the North Pacific Ocean subsequently colonized the Japan Sea and occupied the vacant niches of the deep basins. Between 10 and 8 ka the foraminifer compensation level (FCL) gradually rose to a depth shallower than 1000 m, and bottom conditions changed from dysaerobic to oxic. At 10 ka the warm Tsushima Current started to flow into the Japan Sea through the Tsushima Strait to establish the modern oceanographic regime which has existed since 8 ka. The eustatic sea level during the last glacial maximum was above the sill depths (130 m) of the Tsushima and Tsugaru straits, assuming that tectonic movements at these straits were negligible for the last 20 ka.

Changes in carbon and sulfur cycling over geologic time may have caused considerable modification of atmospheric and oceanic composition and climate. Here we calculate pyrite sulfur (S_py) and organic carbon (C_org) burial rates from recently improved Cenozoic stable isotope records, and from these rates we infer global changes in C_org burial environments. Given predominantly normal shelf‐delta organic carbon burial, the global S_py burial flux should be coupled to C_org burial. However, we find that the major early Cenozoic peak in C_org burial coincides with a minimum in S_py burial. Although the calculated magnitude of variations in global pyrite burial flux is sensitive to our assumptions about the concentration of sulfate in paleoseawater, a non‐steady‐state isotope mass balance model indicates very low S_py burial rates during the Paleocene and a dramatic increase starting near the Paleocene‐Eocene boundary, dropping off to a fairly constant Cenozoic rate beginning in the middle Eocene. High C_org/S_py burial ratios (C/S mole ratio ≈15–30) coinciding with the Paleocene carbon isotope maximum most likely reflect enhanced accumulation of terrestrial organic carbon in Paleocene terrestrial swamps. We suggest that rapid burning of accumulated Paleocene terrestrial organic carbon could have significantly contributed to the short‐lived negative carbon isotope excursion at the Paleocene‐Eocene boundary in addition to or possibly even as an alternative to release of gas hydrates from the continental slopes. An early Eocene minimum in calculated C_org/S_py burial ratios (C/S mole ratio ≈2–4) suggests that the predominant locus of organic carbon burial shifted to euxinic environments in a warm early Eocene ocean.

Surface and deepwater paleoclimate records in Irminger Sea core SO82‐5 (59°N, 31°W) and Icelandic Sea core PS2644 (68°N, 22°W) exhibit large fluctuations in thermohaline circulation (THC) from 60 to 18 calendar kyr B.P., with a dominant periodicity of 1460 years from 46 to 22 calendar kyr B.P., matching the Dansgaard‐Oeschger (D‐O) cycles in the Greenland Ice Sheet Project 2 (GISP2) temperature record [Grootes and Stuiver, 1997]. During interstadials, summer sea surface temperatures (SST_su) in the Irminger Sea averaged to 8°C, and sea surface salinities (SSS) averaged to ∼36.5, recording a strong Irminger Current and Atlantic THC. During stadials, SST_su dropped to 2°–4°C, in phase with SSS drops by ∼1–2. They reveal major meltwater injections along with the East Greenland Current, which turned off the North Atlantic deepwater convection and hence the heat advection to the north, in harmony with various ocean circulation and ice models. On the basis of the IRD composition, icebergs came from Iceland, east Greenland, and perhaps Svalbard and other northern ice sheets. However, the southward drifting icebergs were initially jammed in the Denmark Strait, reaching the Irminger Sea only with a lag of 155–195 years. We also conclude that the abrupt stadial terminations, the D‐O warming events, were tied to iceberg melt via abundant seasonal sea ice and brine water formation in the meltwater‐covered northwestern North Atlantic. In the 1/1460‐year frequency band, benthic δ¹⁸O brine water spikes led the temperature maxima above Greenland and in the Irminger Sea by as little as 95 years. Thus abundant brine formation, which was induced by seasonal freezing of large parts of the northwestern Atlantic, may have finally entrained a current of warm surface water from the subtropics and thereby triggered the sudden reactivation of the THC. In summary, the internal dynamics of the east Greenland ice sheet may have formed the ultimate pacemaker of D‐O cycles.

The trophic resource continuum (TRC) in euphotic zones of the world's oceans is the spectrum of conditions from the richest runoff and upwelling areas to the most nutrient deficient subtropical seas. That spectral range has likely varied through geologic history as more efficient phytoplankton taxa evolved, as oceanic circulation rates changed, and in response to oceanic‐mixing events. Reduced rates of oceanic turnover should produce regionally intensified eutrophy and oligotrophy, that is, TRC expansion. Because oceanic waters as a whole are relatively nutrient rich, increased rates of oceanic turnover should prevent extremely eutrophic and extremely oligotrophic conditions from developing, causing TRC contraction. Global biotic diversities in the oceans are strongly influenced by communities in oligotrophic environments. Nutrient deficient waters are more stable and more transparent than richer waters, permitting higher degrees of specialization. Sparse nutrient supplies necessitate smaller phytoplankton and longer, more complex food chains. Preservation potential of oligotrophic biotas is also greater because calcareous taxa are more prevalent and because calcium carbonate is less apt to be bioeroded or dissolved. Fluctuations in the TRC should profoundly affect diversity of habitats within the euphotic zone of the oceans. The evolutionary response to a TRC expansion, diversification, would occur in a geologic time frame of millions of years, while evolutionary responses to a TRC contraction, including extinction, could occur in a biological time frame of generations. A simple model indicates that a reduction in the trophic resource continuum, from expansion during a major marine transgression, down to modern levels could result in loss of 40% of the euphotic habitats. How fluctuations in the TRC control diversity of euphotic habitats in the oceans provides an hypothesis by which to interpret global taxonomic diversity cycles, as well as taxonomically selective extinctions, changes in species dominance, and some morphologic trends.