Proxy For Export Production Research Articles

Introduction of isotopically light barium from the Rainbow hydrothermal system into the deep Atlantic Ocean

The marine barium (Ba) cycle is closely connected to the short-timescale carbon cycle, and Ba serves as a valuable paleo proxy for export production, ocean alkalinity, and terrestrial inputs. However, the marine Ba budget is poorly constrained, particularly regarding the fluxes of hydrothermally sourced Ba, which hinders our understanding of the Ba cycle and use of Ba-based proxies. Recent studies have suggested a modern source-sink imbalance of Ba isotopes in the global ocean, with sources being overall isotopically heavier than the sinks, and the hydrothermal Ba inputs were considered isotopically heavy sources.In this study, we present the first investigation of Ba and its isotopes in a non-buoyant hydrothermal plume based on dissolved and particulate samples collected from the Rainbow hydrothermal vent field on the Mid-Atlantic Ridge. Our data reveal strong hydrothermal signals at near-field stations, as evidenced by helium isotopes, accompanied by elevated concentrations of dissolved and particulate Ba. Dissolved Ba isotope compositions (δ138Ba) in hydrothermally influenced deep waters (∼0.3 %0) are lighter than at similar depths of far-field stations (∼0.45 %0) in the Atlantic Ocean. The concentrations and isotopic compositions of dissolved and labile particulate Ba in the non-buoyant hydrothermal plume can be explained by conservative mixing between a Ba-enriched hydrothermal component and North Atlantic Deep Water. By extrapolating the correlations to the vent fluid endmember, our results suggest that there is negligible removal of Ba, and insignificant modification of Ba isotopic signatures, from the vent fluid endmember to the non-buoyant hydrothermal plume. This indicates that the Rainbow hydrothermal system introduces isotopically light Ba (−0.17 ± 0.05 %0) to the deep Atlantic Ocean. We estimate that global hydrothermal inputs of Ba are 4.6 ± 2.2 Gmol/yr. These observations highlight the potential of hydrothermal Ba to be an isotopically light source component of the marine Ba isotope budget.

Increased export production during recovery from the Paleocene–Eocene thermal maximum constrained by sedimentary Ba isotopes

The Paleocene–Eocene thermal maximum (PETM; ∼56 Ma) was a transient global warming event associated with a huge perturbation to the global carbon cycle. Changes in marine biological productivity may have contributed to the rapid recovery from this climate change event, by driving the burial of inorganic and organic carbon. Disagreement between proxy reconstructions, however, makes the response of biological productivity to climatic changes experienced during the PETM uncertain. Accumulation of non-detrital barium (Ba) in marine sediments is a commonly used proxy for export production. This proxy however can be compromised by artifacts resulting from dilution and changes in barite preservation, issues that have been debated for its application to sediments deposited during the PETM. Here we present a new approach to address these limitations, by combining non-detrital Ba accumulation with Ba isotope data for marine PETM sediments. Observed positive correlation between these variables is consistent with their control by local changes in export production. These results help resolve previous discrepancies between productivity reconstructions, and indicate export production at sites in the Southern Ocean and South Atlantic decreased or remained unchanged following the PETM onset, followed by an increase to maximum values in the PETM recovery period. This increase in export production coincides with elevated carbonate accumulation rates, representing an important mode of carbon sequestration. These new constraints therefore support the idea that increased production and export of calcifying nannoplankton, perhaps driven by changes in ocean stratification and/or terrestrial runoff, played an important role in rapid recovery from the PETM. This work also demonstrates the utility of sedimentary Ba isotope compositions for understanding past changes in the marine carbon cycle.

Oceanic fronts control the distribution of dissolved barium in the Southern Ocean

The globally-observed relationship between oceanic barium and the macronutrient silicic acid results from the shared influence of large-scale ocean circulation and mixing on the two elements, and the inherent link between barium and organic matter formation and dissolution. A detailed examination of deviations from barium-silicon correlations can reveal variations in non-conservative processes within the marine barium cycle. Here, we present a high-resolution dataset of dissolved barium and macronutrients from the Drake Passage and the Scotia and Weddell Seas. Our new results highlight the influence of Southern Ocean frontal zones on barium cycling and the deviations of barium and macronutrient distributions as a result of spatial variations in phytoplankton assemblages and in barite formation processes. These new data also reinforce findings that water mass mixing and ocean circulation, in particular the location of oxygen minima, play a key role in barium distribution. Our findings have implications for the use of sedimentary barium as a proxy for export production, which may be complicated by physical water circulation changes or shifts in plankton community structure. Corrigendum: https://doi.org/10.1016/j.marchem.2018.09.004

Controls on the barium isotope compositions of marine sediments

The accumulation of barium (Ba) in marine sediments is considered to be a robust proxy for export production, although this application can be limited by uncertainty in BaSO4 preservation and sediment mass accumulation rates. The Ba isotope compositions of marine sediments could potentially record insights into past changes in the marine Ba cycle, which should be insensitive to these limitations, enabling more robust interpretation of sedimentary Ba as a proxy. To investigate the controls on the Ba isotope compositions of marine sediments and their potential for paleo-oceanographic applications, we present the first Ba isotope compositions results for sediments, as well as overlying seawater depth profiles collected in the South Atlantic. Variations in Ba isotope compositions of the sediments predominantly reflect changes in the relative contributions of detrital and authigenic Ba sources, with open-ocean sediments constraining the isotope composition of authigenic Ba to be δBa138/134≈+0.1‰. This value is consistent with the average isotope composition inferred for sinking particulate Ba using simple mass balance models of Ba in the overlying water column and is hypothesized to reflect the removal of Ba from the upper water column with an associated isotopic fractionation of ΔBadiss-part138/134≈+0.4 to +0.5. Perturbations to upper ocean Ba cycling, due to changes in export production and the supply of Ba via upwelling, should therefore be recorded by the isotope compositions of sedimentary authigenic Ba. Such insights will help to improve the reliable application of Ba accumulation rates in marine sediments as a proxy for past changes in export production.

Ocean dynamics, not dust, have controlled equatorial Pacific productivity over the past 500,000 years

Biological productivity in the equatorial Pacific is relatively high compared with other low-latitude regimes, especially east of the dateline, where divergence driven by the trade winds brings nutrient-rich waters of the Equatorial Undercurrent to the surface. The equatorial Pacific is one of the three principal high-nutrient low-chlorophyll ocean regimes where biological utilization of nitrate and phosphate is limited, in part, by the availability of iron. Throughout most of the equatorial Pacific, upwelling of water from the Equatorial Undercurrent supplies far more dissolved iron than is delivered by dust, by as much as two orders of magnitude. Nevertheless, recent studies have inferred that the greater supply of dust during ice ages stimulated greater utilization of nutrients within the region of upwelling on the equator, thereby contributing to the sequestration of carbon in the ocean interior. Here we present proxy records for dust and for biological productivity over the past 500 ky at three sites spanning the breadth of the equatorial Pacific Ocean to test the dust fertilization hypothesis. Dust supply peaked under glacial conditions, consistent with previous studies, whereas proxies of export production exhibit maxima during ice age terminations. Temporal decoupling between dust supply and biological productivity indicates that other factors, likely involving ocean dynamics, played a greater role than dust in regulating equatorial Pacific productivity.

Evaluating Cenozoic equatorial sediment deposition anomalies for potential paleoceanographic and Pacific plate motion applications

If equatorial sediments form characteristic deposits around the equator, they may help to resolve the amount of northwards drift of the Pacific tectonic plate. Relevant to this issue, it has been shown that 230Th has been accumulating on the equatorial seabed faster than its production from radioactive decay in the overlying water column during the Holocene (Marcantonio et al. in Paleoceanography 16:260–267, 2001). Some researchers have argued that this reflects the deposition of particles with adsorbed 230Th carried by bottom currents towards the equator (“focusing”). If correct, this effect may combine with high pelagic productivity, which is also centered on the equator, to yield a characteristic signature of high accumulation rates marking the paleoequator in older deposits. Here we evaluate potential evidence that such an equatorial feature existed in the geological past. Seismic reflection data from seven meridional transects suggest that a band of equatorially enhanced accumulation of restricted latitude was variably developed, both spatially and temporally. It is absent in the interval 14.25–20.1 Ma but is well developed for the interval 8.55–14.25 Ma. We also examined eolian dust accumulation rate histories generated from scientific drilling data. A dust accumulation rate anomaly near the modern equator, which is not obviously related to the inter-tropical convergence zone, is interpreted as caused by focusing. Accumulation rates of Ba and P2O5 (proxies of export production) reveal a static equatorial signature, which suggests that the movement of the Pacific plate over the period 10–25 Ma was modest. The general transition from missing to well-developed focusing signatures around 14.25 Ma in the seismic data coincides with the mid-Miocene development of the western boundary current off New Zealand. This current supplies the Pacific with deep water from Antarctica, and could therefore imply a potential paleoceanographic or paleoclimatic origin. At 10.05–14.25 Ma, the latitudes of the seismic anomalies are up to ~2° different from the paleoequator predicted by Pacific plate-hotspot models, suggesting potentially a small change in the hotspot latitudes relative to the present day (although this inference depends on the precise form of the deposition around the equator). The Ba and P2O5 anomalies, on the other hand, are broadly compatible with plate models predicting slow northward plate movement over 10–25 Ma.

A quantitative reconstruction of organic matter and nutrient diagenesis in Mediterranean Sea sediments over the Holocene

A multicomponent diagenetic model was developed and applied to reconstruct the conditions under which the most recent sapropel, S1, was deposited in the eastern Mediterranean Sea. Simulations demonstrate that bottom waters must have been anoxic and sulphidic during the formation of S1 and that organic matter deposition was approximately three times higher than at present. Nevertheless, most present day sediment and pore water profiles — with the exception of pyrite, iron oxyhydroxides, iron-bound phosphorus and phosphate — can be reproduced under a wide range of redox conditions during formation of S1 by varying the depositional flux of organic carbon. As a result, paleoredox indicators (e.g., C org:S ratio, C org:P org ratio, trace metals) are needed when assessing the contribution of oxygen-depletion and enhanced primary production to the formation of organic-rich layers in the geological record. Furthermore, simulations show that the organic carbon concentration in sediments is a direct proxy for export production under anoxic bottom waters. The model is also used to examine the post-depositional alteration of the organic-rich layer focussing on nitrogen, phosphorus, and organic carbon dynamics. After sapropel formation, remineralisation is dominated by aerobic respiration at a rate that is inversely proportional to the time since bottom waters became oxic once again. A sensitivity analysis was undertaken to identify the most pertinent parameters in regulating the oxidation of sapropels, demonstrating that variations in sedimentation rate, depositional flux of organic carbon during sapropel formation, bottom water oxygen concentration, and porosity have the largest impact. Simulations reveal that sedimentary nutrient cycling was markedly different during the formation of S1, as well as after reoxygenation of bottom waters. Accumulation of organic nitrogen in sediments doubled during sapropel deposition, representing a significant nitrogen sink. Following reventilation of deep waters, N 2 production by denitrification was almost 12 times greater than present day values. Phosphorus cycling also exhibits a strong redox sensitivity. The benthic efflux of phosphate was up to 3.5 times higher during the formation of S1 than at present due to elevated depositional fluxes of organic matter coupled with enhanced remineralisation of organic phosphorus. Reoxygenation of bottom waters leads to a large phosphate pulse to the water column that declines rapidly with time due to rapid oxidation of organic material. The oxidation of pyrite at the redox front forms iron oxyhydroxides that bind phosphorus and, thus, attenuate the benthic phosphate efflux. These results underscore the contrasting effects of oxygen-depletion on sedimentary nitrogen and phosphorus cycling. The simulations also confirm that the current conceptual paradigm of sapropel formation and oxidation is valid and quantitatively coherent.

Enhanced marine productivity in the Kuroshio region off Shikoku during the last glacial period inferred from the accumulation and carbon isotopes of sedimentary organic matter

AbstractWe obtained marine primary production records from cores MD01‐2422 and KH06‐3‐PC8 in the modern Kuroshio flow region in the western North Pacific for the last 35 ka. The organic carbon content of marine sediments is commonly used as a proxy for export production from the surface ocean. Most sedimentary organic carbon was derived from marine organisms, because carbon isotopes of organic matter (δ13Corg) in sediments were approximately −21‰, which is close to the typical marine δ13Corg value. The quantities of marine and terrestrial organic matter were reconstructed from the mass balance of δ13Corg in bulk total organic carbon. The accumulation of marine organic carbon, and presumably marine production, was enhanced during the latter half of the last glacial period in the subtropical gyre in the western North Pacific. Terrestrial organic matter varied synchronously with marine organic carbon changes, suggesting that both increased in late Marine Isotope Stage (MIS) 2. We conclude that in addition to aeolian dust supplied by the East Asia monsoon, the lateral transport of nutrients from the East China Sea to the subtropical gyre as a result of a sea‐level rise at the last deglaciation played an essential role in enhancing palaeoproductivity during the last glacial period. Copyright © 2009 John Wiley & Sons, Ltd.

Primary productivity response to Heinrich events in the North Atlantic Ocean and Norwegian Sea

The last glacial was punctuated by several massive ice sheet surges into the North Atlantic that impacted surface water hydrology especially where icebergs melted. However, the links between variations in surface water hydrology and surface water productivity during these Heinrich events (HEs) remain uncertain. To address this issue, diatoms and organic carbon were examined across Heinrich event 1 (HE 1) and Heinrich event 4 (HE 4) in seven sediment cores spanning 40°N to 63°N latitude. Our results show low diatom abundances during HEs, consistent with decreased surface water productivity. Diatom dilution by increased sediment flux was tested by normalizing diatom abundance to a constant 230Th flux. Although the particle rain rate was enhanced during HEs, this does not explain the sharp drop in diatoms. During HE 4, surface productivity decreased at all latitudes examined, probably because of strong, year‐round stratification. The same inferred changes occurred during HE 1 within the area of maximum iceberg melting. However, at northern latitudes (above 50°N) the summer insolation increase of the glacial termination drove increased surface productivity during the whole period, including HE 1. Marine organic carbon, taken as independent proxy for export production, supports the diatom data. Trends shown by the productivity proxies evolve generally in parallel with the hydrographic proxies, with an increase in productivity when sea surface temperature increases.

On the relationships between primary, net community, and export production in subtropical gyres

It has been proposed that net primary production (NPP), net community production (NCP), particulate organic carbon export (Phi(POC)) and the relationships among them are governed by local environmental conditions that favor either a microbially dominated assemblage leading to a regeneration loop (low ratio of (Phi(POC) to NPP) or a system dominated by large plankton with export pathway characteristics (high ratio of (Dpoc to NPP). We analyze more than 10 years of data from two subtropical time-series stations (Hawaii Ocean Times-series (HOT) in the North Pacific, and Bermuda Atlantic Time-Series (BATS) in the North Atlantic) in order to investigate this regeneration loop versus export pathway hypothesis and in particular to test the idea that the switch between the two is controlled by enhanced input of nutrients. In the decadal long-term mean, the relationships between NPP, (Dpoc and NCP, which we take here as a proxy for export production, reveal export pathway characteristics at BATS, while HOT is dominated by the regeneration loop. This difference is consistent with the stronger seasonal forcing at BATS and the resulting higher new nutrient input. However, these characteristics are only valid for parts of the year. Especially at BATS, the export pathway exists only in spring and the system reverts to a regeneration loop in summer and fall. This is consistent with our hypothesis given the strong summer-time stratification and the resulting low levels of new nutrient input. On interannual time-scales, we find little evidence for statistically significant alterations of the long-term mean characteristics, a finding we ascribe to a combination of limited magnitude of forcing, length of the data records, and possibly an inherent lack of predictability. A comparison of our results for the ratio between NCP and NPP (e-ratio) and the ratio between Phi(POC) and NPP (pe-ratio) with those predicted by the models of Laws et al. [Temperature effects on export production in the open ocean. Global Biogeochemical Cycles 14(4), 1231-1246] and Dunne et al. [Empirical and mechanistic models for particle export ratio. Global Biogeochemical Cycles 19, GB40261 respectively, show reasonable agreement for the long-term mean, but these models fail to capture the observed interannual variability in these ratios. (c) 2006 Elsevier Ltd. All rights reserved.

Organic carbon accumulation over the last 16 kyr off Vancouver Island, Canada: evidence for increased marine productivity during the deglacial

The organic carbon content of marine sediments is commonly used as a proxy for export production. However, in continental margin sediments a large fraction of the organic matter may be of terrestrial origin, and it is necessary to correct the total organic carbon data accordingly. Radiocarbon dating of bulk organic carbon, organic geochemistry and isotope data (δ 13C and δ 15N) are used here to characterize the type of organic matter present in Core JT96-09 collected at a water depth of 920 m on the slope off Western Canada. The quantities of marine and terrestrial organic carbon are then estimated using the δ 13C data. The 16 kyr record obtained from Core JT96-09 suggests that accumulation of total organic carbon was highest during the late glacial and deglacial, but geochemical data indicate that as much as 70% of this carbon is terrestrial in origin. When the palaeo-record is corrected for this terrigenous input it is observed that accumulation of marine organic matter, and presumably marine export production, increased at the end of the last glacial contemporaneous with the Bølling, and that it peaked during the Allerød. Data from other palaeoproductivity proxies (i.e., bio-barium, opal and alkenones) also indicate relatively high productivity during the deglacial. These results indicate a return to modern upwelling conditions and high marine export production at ∼14.3 calendar kyr BP and a period of enhanced upwelling, relative to the present, during the Allerød.

Patterns of export production in the eastern equatorial Pacific over the past 130,000 years

Productivity at six core locations in the eastern equatorial Pacific (EEP) was reconstructed with a benthic foraminiferal transfer function. The core records show strong regionality, especially where affected by Peru margin upwelling of deeper Equatorial Undercurrent Water (EUC) (originally coming from the subantarctic). This “Peru margin” record differs from that seen along the equator where divergence leads to shallow upwelling, and it is generally inverse to that seen in cores outside the areas of equatorial upwelling. Principal components analysis shows that the main productivity pattern correlates well to the global oxygen isotope record and has lowest values during isotope stages 2 and 4. In addition to this, equatorial cores show a higher frequency pattern of variation which becomes much more pronounced during MIS 3 and 2. The reconstructions based on benthic foraminifera were tested against those from nonaccumulation rate based inorganic chemical proxies of export production. These were found to correlate well in the region influenced by Peru upwelling, and also to share common features for sites along the equator. All the nonaccumulation rate based paleotracers are consistent with one another and differ from accumulation rate derived proxies. The differences between the two classes of paleotracers may result from uncertainties in calculating actual biogenic fluxes since 230Th‐normalized results conform more to those we obtained. Analysis of planktonic carbon isotope values for the EEP, and their comparison to the record of the Pacific subantarctic, indicates that the subantarctic contribution to the EUC was reduced during MIS 3 and 2.

Pliocene export production and terrigenous provenance of the Southern Cape Basin, southwest African margin

We have analyzed 33 Pliocene bulk sediment samples from Ocean Drilling Program Site 1085 in the Cape Basin, located offshore of western Africa in the Angola–Benguela Current system, for 17 major and trace elements, and interpreted their associations and temporal variations in the context of an allied data set of CaCO 3, opal, and C org. We base our interpretations on elemental ratios, accumulation rates, inter-element correlations, and several multi-element statistical techniques. On the basis of qualitative assessment of downhole changes in the distributions of P and Ba, utilized as proxies of export production, we conclude that highs in bulk and biogenic accumulation that occur at 3.2 Ma, 3.0 Ma, 2.4 Ma, and 2.25 Ma were caused by increases in export production as well as terrigenous flux, and record a greater sequestering of organic matter during these time periods. Studies of refractory elements and other indicator proxies (SiO 2, Al 2O 3, TiO 2, Fe 2O 3, MgO, V, Cr, Sr, and Zr) strongly suggest that the terrigenous component of the bulk sediment is composed of two compositional end-members, one being ‘basaltic’ in composition and the other similar to an ‘average shale’. The basaltic end-member comprises approximately 10–15% of the total bulk sediment and its presence is consistent with the local geology of source material in the drainage basin of the nearby Orange River. The increase in bulk accumulation at 2.4 Ma appears to reflect a greater relative increase in basaltic input than the relative increase in shale-type input. Although studies such as this cannot precisely identify the transport mechanisms of the different terrigenous components, these results are most consistent with variations in sea level (and associated changes in shelf geometry and fluvial input) being responsible for the changing depositional conditions along the Angolan Margin during this time period.

Export production and terrigenous matter in the Central Equatorial Pacific Ocean during interglacial oxygen isotope Stage 11

This study addresses changes in the absolute magnitude and spatial geometry of particle flux and export production in a meridional transect across the central equatorial Pacific Ocean's upwelling system during oxygen isotope Stage 11 and Stage 12 and compares these time periods to the current Holocene interglacial system. Temporal and spatial variability in several chemical proxies of export production, and in particular the distributions of Ba, scavenged Al, and P, are studied in a suite of sediment cores gathered along a cross-equator transect at 5°S, 2°S, 0°, 2°N, and 4°N. Because this latitudinal range preserves strong gradients in biogenic particle flux in the modern equatorial Pacific Ocean, we are able to assess variations in the relative magnitude of export production as well as the meridional width of the equatorial system through the late Quaternary glacial/interglacial cycles. During interglacial oxygen isotope Stage 11 the chemical proxies each indicate lower particle flux and export production than during Stage 12. These records are consistent throughout the transect during this time period, but geographic narrowing (during the interglacial) and widening (during the glacial) of the meridional gradient also occurs. Although carbonate concentration varies dramatically through glacial/interglacial cycles at all latitudes studied, the productivity proxies record only minimal glacial/interglacial change at 5°S and 4°N, indicating that the carbonate minima at these latitudes is controlled dominantly by dissolution rather than production. The chemical data indicate that although the spatial geometry of the system during Stages 11 and 12 indicates maximum productivity at the equator during both glacial and interglacial conditions, the absolute magnitude of export production integrated from 5°S to 4°N during Stage 11 was 25–50% less than during Stage 12, and also was 25–50% less than it is now.

200 000 yr diatom records from Atlantic upwelling sites reveal maximum productivity during LGM and a shift in phytoplankton community structure at 185 000 yr

Time series of diatom accumulation rates (DAR) and assemblage composition for the last 200 000 yr from the Atlantic major coastal upwelling loci of the eastern boundary currents and eastern Equator are compared together with independent proxies of export production, to upper ocean productivity and to nutrient concentration and utilization. Fluctuations in DAR are concomitant at all sites and in good correspondence to the other independent productivity proxies indicating glacial periods as the most productive times of the past 200 000 yr in both hemispheres of the eastern Atlantic. Maximum productivity, about one order of magnitude higher than at present, occurred at the last glacial maximum (LGM). Despite the ‘Atlantic’ similarities in both coastal and equatorial upwelling types, important regional and site specific variations emerge. In the southern Hemisphere site, a contradiction of proxies, diatoms and Corg, is noted at the base of stage 6 (185 000 yr). The good correlation found between the Corg and the benthic foraminifera fluxes is interpreted as a reflection of a shift in the phytoplankton community structure related to either a change in the N:P:Si ratio, or similar nutrient conditions associated with increased water column stratification.

Late-Quaternary changes in productivity of the Southern Ocean

Paleoceanographic records based on new proxies of export production have been constructed for the South Atlantic sector of the Southern Ocean. A radionuclide-ratio proxy of particle flux ( 10 Be/ 230 Th ) and the accumulation rate of authigenic uranium, which responds to the flux of organic carbon to the sea bed, both indicate a dramatic increase, compared to the present, in the export production of the Subantarctic zone (approximately the region between the present-day positions of the Subtropical Convergence and the Antarctic Polar Front) during glacial periods. If the South Atlantic is representative of the entire Southern Ocean, then export production in the Southern Ocean during the Last Glacial Maximum was substantially greater than at present. Previous studies, focusing on the burial of biogenic opal, failed to recognize the glacial increase in export production of the Southern Ocean because of a strong non-linearity between accumulation rates of opal and of organic carbon.

Latitudinal variations in benthic processes in the abyssal equatorial Pacific: control by biogenic particle flux

The equatorial Pacific forms a band of high, globally significant primary production. This productivity drops off steeply with distance from equatorial upwelling, yielding large latitudinal gradients in biogenic particle flux to the abyssal seafloor. As part of the US JGOFS Program, we studied the translation of these particle-flux gradients into the benthic ecosystem from 12°S to 9°N along 135–140°W to evaluate their control of key benthic processes, and to evaluate sediment proxies of export production from overlying waters. In October–December 1992 the remineralization rates of organic carbon, calcium carbonate and biogenic opal roughly matched the rain rates of these materials into deep sediment traps, exhibiting peak values within 3° of the equator. Rates of bioturbation near the equator were about ten-fold greater than at 9°N, and appeared to exhibit substantial dependence on particulate-organic-carbon flux, tracer time scale (i.e. age-dependent mixing), and pulsed mixing from burrowing urchins. Organic-carbon degradation within sediments near the equator was dominated by a very labile component (reaction rate constant, k approximately 15 per year) that appeared to be derived from greenish phytodetritus accumulated on the seafloor. Organic-carbon degradation at the highest latitudes was controlled by a less reactive component, with a mean k of approximately 0.075 per year. Where measured, megafaunal and macrofaunal abundances were strongly correlated with annual particulate-organic carbon flux; macrofaunal abundance in particular might potentially serve as a proxy for export production in low-energy abyssal habitats. Sedimentary microbial biomass also was correlated with the rain rate of organic carbon, but less strongly than larger biota and on shorter time scales (i.e. approximately 100 days). We conclude that the vertical flux of biogenic particlues exerts tight control on the nature and rates of benthic biological and chemical processes in the abyssal equatorial Pacific, and suggest that global changes in productivity on decadal or greater time scales could yield profound changes in deep-sea benthic ecoystems.

Remineralization of particulate authigenic trace metals in the middle Atlantic Bight: Implications for proxies of export production

Samples collected by time-series sediment traps deployed in the Middle Atlantic Bight were studied to better understand the formation, and preservation, of particulate authigenic forms of trace metals (Cu, Ni, Ba) That hold potential to serve as proxies in the sedimentary record of past changes in the flux of biogenic detritus sinking from the surface ocean into the deep sea (export production). Particulate biogenic and authigenic phases are extremely labile, as evidenced by the observation that as much as 70% of the particulate fluxes of organic carbon and of certain metals (Cu, Ni, and Mn), and up to 25% of the particulate fluxes of authigenic Ba and of opal collected by sediment traps are released rapidly into solution during the time period between particle collection and trap retrieval. Further remineralization on the seabed reduces concentrations of authigenic Cu and Ni in surface sediments below the limit of detection. Approximately 80% of authigenic Ba is remineralized during early diagenesis on the seabed, much more than is expected for conditions of high sediment mass accumulation rate that exist in the study area. Extensive remineralization during early diagenesis, combined with large corrections required to remove the aluminosilicate contribution to the concentrations of Cu, Ni, and Ba in sediments, preclude the successful use of down-core profiles of these trace metals to reconstruct past changes in export production of the Middle Atlantic Bight. Similar problems are likely to plague paleoproductivity reconstructions in other ocean-margin regions, or wherever high fluxes of aluminosilicate phases occur.