Biogenic Barium Research Articles

Diverse patterns of ocean export productivity change across the Cretaceous‐Paleogene boundary: New insights from biogenic barium

One of the best‐studied aspects of the K‐Pg mass extinction is the decline and subsequent recovery of open ocean export productivity (e.g., the flux of organic matter from the surface to deep ocean). Some export proxies, including surface‐to‐deep water δ13C gradients and carbonate sedimentation rates, indicate a global decline in export productivity triggered by the extinction. In contrast, benthic foraminiferal and other geochemical productivity proxies suggest spatially and temporally heterogeneous K‐Pg boundary effects. Here we address these conflicting export productivity patterns using new and compiled measurements of biogenic barium. Unlike a previous synthesis, we find that the boundary effect on export productivity and the timing of recovery varied considerably between different oceanic sites. The northeast and southwest Atlantic, Southern Ocean, and Indian Ocean records saw export production plummet and remain depressed for 350 thousand to 2 million years. Biogenic barium and other proxies in the central Pacific and some upwelling or neritic Atlantic sites indicate the opposite, with proxies recording either no change or increased export production in the early Paleocene. Our results suggest that widespread declines in surface‐to‐deep ocean δ13C do not record a global decrease in export productivity. Rather, independent proxies, including barium and other geochemical proxies, and benthic community structure, indicate that some regions were characterized by maintained or rapidly recovered organic flux from the surface ocean to the deep seafloor, while other regions had profound reductions in export productivity that persisted long into the Paleocene.

A comparison of early Paleogene export productivity and organic carbon burial flux for Maud Rise, Weddell Sea, and Kerguelen Plateau, south Indian Ocean

[1] Marine biological productivity has been invoked as a possible climate driver during the early Paleogene through its potential influence on atmospheric carbon dioxide concentrations. However, the relationship of export productivity (the flux of organic carbon (C) from the surface ocean to the deep ocean) to organic C burial flux (the flux of organic C from the deep ocean that is buried in marine sediments) is not well understood. We examine the various components involved with atmosphere-to-ocean C transfer by reconstructing early Paleogene carbonate and silica production (using carbonate and silica mass accumulation rates (MARs)); export productivity (using biogenic barium (bio-Ba) MARs); organic C burial flux (using reactive phosphorus (P) MARs); redox conditions (using uranium and manganese contents); and the fraction of organic C buried relative to export productivity (using reactive P to bio-Ba ratios). Our investigations concentrate on Paleocene/Eocene sections of Sites 689/690 from Maud Rise and Site 738 from Kerguelen Plateau. In both regions, export productivity, organic C burial flux, and the fraction of organic C buried relative to export productivity decreased from the Paleocene/early Eocene to the middle Eocene. A shift is indicated from an early Paleogene two-gyre circulation in which nutrients were not efficiently recycled to the surface via upwelling in these regions, to a circulation more like the present day with efficient recycling of nutrients to the surface ocean. Export productivity was enhanced for Kerguelen Plateau relative to Maud Rise throughout the early Paleogene, possibly due to internal waves generated by the plateau regardless of gyre circulation.

Productivity feedback did not terminate the Paleocene-Eocene Thermal Maximum (PETM)

Abstract. The Paleocene-Eocene Thermal Maximum (PETM) occurred approximately 55 million years ago, and is one of the most dramatic abrupt global warming events in the geological record. This warming was triggered by the sudden release of thousands of gigatons of carbon into the atmosphere and is widely perceived to be the best analogue for current anthropogenic climate change. Yet, the mechanism of recovery from this event remains controversial. A massive increase in the intensity of the marine biological pump ("productivity feedback") has been suggested to cause a drawdown of atmospheric CO2 and subsequent carbon sequestration in the ocean. A re-evaluation of the "productivity feedback hypothesis", based on biogenic barium mass accumulation rates (Ba-MARs) for a site in the Southern Ocean, finds that any increase in export production lagged the initial carbon release by at least ~70 000 years. This implies that export production did not facilitate rapid removal of excess carbon from the atmosphere. Thus, the most likely mechanism for carbon removal appears to be silicate weathering, which occurred at much slower rates than previously assumed.

A pervasive link between Antarctic ice core and subarctic Pacific sediment records over the past 800 kyrs

Recently developed XRF core-scanning methods permit paleoceanographic reconstructions on timescales similar to those of ice-core records. We have investigated the distribution of biogenic barium (Ba/Al), opal and carbonate (Ca/Al) in a sediment core retrieved from the abyssal subarctic Pacific (ODP 882, 50°N, 167°E, 3244 m) over an interval that spans the full length of the EPICA Dome C (EDC) ice-core record. Ba/Al and biogenic opal show a strong resemblance to the EDC δD and CO 2, with generally high concentrations during interglacials and lower values during ice ages of the past 800 kyrs. The sedimentary Ba/Al and biogenic opal are most easily interpreted as indicating a reduced sinking flux of organic matter from the surface ocean during cold periods. The Ba/Al maxima during peak interglacials are accompanied by transient Ca/Al peaks in these otherwise carbonate-devoid sediments, which are best explained by a deepening of the calcite lysocline, presumably due to reduced storage of respired CO 2 in the deep North Pacific. For most of the “luke-warm” interglacials noted between 420 and 750 ka in EDC, the Ba/Al peaks in ODP 882 are also lower, further strengthening the evidence for a simple physical link between global climate and the biogeochemistry of the subarctic Pacific.

Palaeoproductivity changes and upwelling variability in the Galicia Mud Patch during the last 5000 years: geochemical and microfloral evidence

The Holocene palaeoclimatic history of the Galician continental shelf has been investigated through the analyses of diatom remains, other siliceous compounds, biogenic silica (BSi) and metals content in a multiproxy approach to a gravity core recovered from the Galicia Mud Patch, NW Iberian Peninsula, covering the last 5000 years. Downcore changes in diatom assemblages composition and abundance reflect changes in diatom production related to long-/short-term variations in climate, regional oceanography, upwelling strength and river influx off the coast of NW Spain and Portugal. Palaeoclimatic variability was related to the relative strengths and position of the Azores High and Iceland Low pressure cells. Metals and microflora fluctuations are interpreted as changes in the riverine influence and upwelling intensity paced by oceanographic, atmospheric and climatic changes. Lack of diatoms between 4700—3300 and 1800—1200 cal. yr BP could be linked to early diagenetic processes taking place in the sediment after burial. Biogenic barium (Baexcess), metals and excess diatom assemblages, show a general increase of marine productivity for the last 1200 cal. yr BP. Between 800 and 500 cal. yr BP high production of the microflora is triggered by influx of river-derived nutrients under conditions of SW winds and storms resulting from a NAO negative-like phase. The biosiliceous and geochemical signatures of sediments from the last 500 cal. yr BP indicate conditions of enhanced upwelling and increased phytoplanktonic production associated with the intensification of northerly winds. Upwelling strengthening in this area, attributed to recent global warming, could provoke an increase in phytoplankton biomass with consequent biological, climatological and socioeconomical impacts. The imprint of anthropic activities has been recorded by the increasing Pb/Al ratios for the last 400 cal. yr BP.

Subarctic Pacific evidence for a glacial deepening of the oceanic respired carbon pool

Measurements of benthic foraminiferal cadmium:calcium (Cd/Ca) have indicated that the glacial–interglacial change in deep North Pacific phosphate (PO 4) concentration was minimal, which has been taken by some workers as a sign that the biological pump did not store more carbon in the deep glacial ocean. Here we present sedimentary redox-sensitive trace metal records from Ocean Drilling Program (ODP) Site 882 (NW subarctic Pacific, water depth 3244 m) to make inferences about changes in deep North Pacific oxygenation – and thus respired carbon storage – over the past 150,000 yr. These observations are complemented with biogenic barium and opal measurements as indicators for past organic carbon export to separate the influences of deep-water oxygen concentration and sedimentary organic carbon respiration on the redox state of the sediment. Our results suggest that the deep subarctic Pacific water mass was depleted in oxygen during glacial maxima, though it was not anoxic. We reconcile our results with the existing benthic foraminiferal Cd/Ca by invoking a decrease in the fraction of the deep ocean nutrient inventory that was preformed, rather than remineralized. This change would have corresponded to an increase in the deep Pacific storage of respired carbon, which would have lowered atmospheric carbon dioxide (CO 2) by sequestering CO 2 away from the atmosphere and by increasing ocean alkalinity through a transient dissolution event in the deep sea. The magnitude of change in preformed nutrients suggested by the North Pacific data would have accounted for a majority of the observed decrease in glacial atmospheric pCO 2.

Seasonal cycle of suspended barite in the mediterranean sea

Biogenic barium (Ba xs) was measured in suspended particles at the DYFAMED site in the northwestern Mediterranean Sea, on a monthly basis between February and June 2003. The barium content of barite (BaSO 4) micro-crystals was investigated using Scanning Electron Microscopy (SEM). Suspended particles were collected by filtration of small volumes of seawater (∼10 L), as well as large volumes up to 2400 L in March and in May. The Ba xs profiles obtained from small-volume filtration display the typical mesopelagic maximum reported by earlier studies at ∼200 m depth, with concentrations up to 595 pmol L −1. In addition, suspended Ba xs was found almost exclusively in the form of micro-crystalline barite, except in February. The Ba xs profiles obtained from large-volume filtration are consistent with the small-volume filtration findings, but reveal a significant Ba xs peak of 1698 pmol L −1 in the surface waters in May. Seasonal sampling at the DYFAMED site shows a net increase in barite concentration during phytoplanktonic blooms, confirming the involvement of biological systems in barite formation, as well as the potential role of barite as a primary productivity tracer. In addition, the coincidence between the mesopelagic barite maximum and the oxygen minimum layer suggests that barite is primarily found at depths of intense remineralization, in agreement with the hypothesis that barite forms within microenvironments of decaying organic matter.

Biogenic barium in surface sediments of the European Nordic Seas

Barium in marine terrigenous surface sediments of the European Nordic Seas is analysed to evaluate its potential as palaeoproductivity proxy. Biogenic Ba is calculated from Ba and Al data using a conventional approach. For the determination of appropriate detrital Ba/Al ratios a compilation of Ba and Al analyses in rocks and soils of the catchments surrounding the Nordic Seas is presented. The resulting average detrital Ba/Al ratio of 0.0070 is similar to global crustal average values. In the southern Nordic Seas the high input of basaltic material with a low Ba/Al ratio is evident from high values of magnetic susceptibility and low Al/Ti ratios. Most of the Ba in the marine surface sediments is of terrigenous and not of biogenic origin. Variability in the lithogenic composition has been considered by the application of regionally varying Ba/Al ratios. The biogenic Ba values are comparable with those observed in the central Arctic Ocean, they are lower than in other oceanic regions. Biogenic Ba values are correlated with other productivity proxies and with oceanographic data for a validation of the applicability in paleoceanography. In the Iceland Sea and partly in the marginal sea–ice zone of the Greenland Sea elevated values of biogenic Ba indicate seasonal phytoplankton blooms. In both areas paleoproductivities may be reconstructed based on Ba and Al data of sediment cores.

Particulate barium fluxes and export production in the northwestern Mediterranean

Biogenic barium, mostly in the barite (BaSO 4) form, has been proposed as a tracer for export production in the ocean. Here we report on biogenic barium (Ba xs) and particulate organic carbon (POC) fluxes from sediment traps deployed at the DYFAMED site in the Northwestern Mediterranean Sea. Ba xs fluxes display average values of 37 ± 45 and 50 ± 58 μg/m 2/d at 200 and 1000 m respectively, and are linearly correlated to POC fluxes (mean values of 7.9 ± 9.3 and 6.8 ± 6.8 mg C/m 2/d at 200 and 1000 m). Export production estimates, calculated using published Ba xs- or POC-based algorithms, all fall below or close to the lower limit of potential export values proposed in the literature. This work clearly demonstrates the usefulness of Ba xs as a tracer of oceanic export production in the Northwestern Mediterranean Sea. However, development of a quantitative export production proxy requires a clear understanding of the underlying cause(s) for the observed spatial variations in the relationship between Ba xs and POC fluxes. The present study confirms that the processes leading to barite formation differ between margin and open-ocean sites and probably account for much of the regional variability in the POC/Ba xs ratio.

Evidence from diatom‐bound nitrogen isotopes for subarctic Pacific stratification during the last ice age and a link to North Pacific denitrification changes

In a piston core from the central Bering Sea, diatom microfossil‐bound N isotopes and the concentrations of opal, biogenic barium, calcium carbonate, and organic N are measured over the last glacial/interglacial cycle. Compared to the interglacial sections of the core, the sediments of the last ice age are characterized by 3‰ higher diatom‐bound δ15N, 70 wt % lower opal content and 1200 ppm lower biogenic barium. Taken together and with constraints on sediment accumulation rate, these results suggest a reduced supply of nitrate to the surface due to stronger stratification of the upper water column of the Bering Sea during glacial times, with more complete nitrate consumption resulting from continued iron supply through atmospheric deposition. This finding extends the body of evidence for a pervasive link between cold climates and polar ocean stratification. In addition, we hypothesize that more complete nutrient consumption in the glacial age subarctic Pacific contributed to the previously observed ice age reduction in suboxia and denitrification in the eastern tropical North Pacific by lowering the nutrient content of the intermediate‐depth water formed in the subpolar North Pacific. In the deglacial interval of the Bering Sea record, two apparent peaks in export productivity are associated with maxima in diatom‐bound and bulk sediment δ15N. The high δ15N in these intervals may have resulted from greater surface nutrient consumption during this period. However, the synchroneity of the deglacial peaks in the Bering Sea with similar bulk sediment δ15N changes in the eastern Pacific margin and the presence of sediment lamination within the Bering Sea during the deposition of the productivity peaks raise the possibility that both regional and local denitrification worked to raise the δ15N of the nitrate feeding Bering Sea surface waters at these times.

Cretaceous black shales as active bioreactors: A biogeochemical model for the deep biosphere encountered during ODP Leg 207 (Demerara Rise)

A transport–reaction model was designed to identify the combination and importance of biogeochemical processes operating in four sites drilled during ODP Leg 207 (Demerara Rise, Equatorial Atlantic). Almost 100 Ma after their deposition, deeply buried Cretaceous black shales still act as active bioreactors in great sediment depths and control the biogeochemical reaction network of the whole sediment column. According to a model calibrated at the four drill sites through inverse modeling techniques, methanogenesis could be identified as a key process that dominates not only organic matter degradation but also sulfate availability through the anaerobic oxidation of methane above the black shales. A complete depletion of sulfate within the black shale sequences promotes the remobilization of biogenic barium that reprecipitates as authigenic barite at the top of the sulfate depletion zone. Temporal dynamics of degradation processes caused continuous shifts of the barite precipitation zone during burial, thus inhibiting the formation of an authigenic barite front or causing the dissolution of earlier formed fronts. Major deviations of pore water sulfate profiles from a linear gradient coincide with depths of decelerated or accelerated transport caused by local porosity minima or maxima. Model-determined reaction rates are by far lower than those found in shallower sediments due to the low metabolic activities that are characteristic for the Deep Biosphere. But even after almost 100 Ma, changing organic matter quality still influences the degradation within the black shale sequences, as it is indicated by model results.

Use of multiproxy records on the Agulhas Ridge, Southern Ocean (Ocean Drilling Project Leg 177, Site 1090) to investigate sub‐Antarctic hydrography from the Oligocene to the early Miocene

Ocean Drilling Program (ODP) Site 1090, on the Agulhas Ridge in the South Atlantic sector of the Southern Ocean, is ideally located to capture changes in Southern Ocean circulation patterns. Using samples taken from cored sediments, we construct multiproxy records of productivity (biogenic barium (Baex), opal, and CaCO3 mass accumulation rates (MARs)), nutrient and organic carbon burial (reactive phosphorus (Pr) MARs), and redox conditions (U and Mn enrichments) to investigate hydrographic conditions associated with climatic shifts from the Oligocene through the early Miocene. Orbitally induced cyclicity in U and Mn enrichments (100 kyr) suggests shifts in deepwater characteristics. However, CaCO3 dissolution coincident with low U and Mn enrichments does not indicate low‐oxygen, corrosive waters similar to modern conditions. These observations indicate that a well‐developed “modern‐type” Antarctic Circumpolar Current (ACC) did not yet exist over the period from 30 to 20 Ma, with two potential consequences: The Southern Ocean was not functioning as a silica trap, permitting a broader distribution of silica that may have facilitated organic carbon burial in the ocean in general, and the lack of a deeply mixing ACC may have facilitated organic carbon burial in the Southern Ocean. Both the relative (high opal MARs coincident with low CaCO3 MARs) and absolute (high Pr MARs) burial of organic carbon suggest a powerful mechanism for pCO2 drawdown.

Settling barium fluxes in the Arabian Sea: Critical evaluation of relationship with export production

Abstract Time series measurement of biogenic and barium fluxes was made using nine sediment traps deployed in the western, central and eastern part of the Arabian Sea with the objective of evaluating barium as a proxy for surface ocean productivity. Our observations show a strong linear correlation between Baexcess fluxes and biogenic opal and organic carbon (Corg) fluxes, indicating a biogenic origin of particulate Ba. However, the correlation between biogenic and Baexcess fluxes is remarkably strong in areas of uniform productivity like the central Arabian Sea, when compared to regions of episodic productivity. The processes that precipitate particulate Ba appear to be less active during periods of high biogenic flux. A large discrepancy is documented between the export flux calculated from the barium-based algorithm [Francois, R., Honjo, S., Manganini, S.J., Ravizza, G.E., 1995. Biogenic barium fluxes to the deep sea: implications for paleoproductivity reconstruction. Global Biogeochemical Cycles 9, 289–303] and from Corg fluxes [Sarnthein, M., Winn, K., Duplessy, J.C., Fontugne, M.R., 1988. Global variations of surface ocean productivity in low and mid latitudes: influence on CO2 reservoirs of the deep ocean and atmosphere during the last 21,000 years. Paleoceanography 3, 361–399] at different depths. However, ∼35% increase in Ba fluxes and a concomitant increase in Ba/Corg ratios are documented between 919 and 2002 m water depth in the western Arabian Sea. This increase may be due either to barite formation within the fecal pellets of mesozooplankton thriving at deeper levels or to scavenging of Ba by Mn oxyhydroxides. Manganese oxyhydroxides seem to act as a prominent scavenging phase for particulate Ba when the Mnexcess content in the settling particles exceeds ∼200 ppm, especially in the western and central Arabian Sea where in situ precipitation of Mn oxides is reported. The estimated preservation efficiency of Ba in the Arabian Sea sediments ranges between 50% and 61%. This is two-fold higher than the global average, suggesting that Ba may be a promising proxy for paleoproductivity estimation.

Glacial/Interglacial Changes in Subarctic North Pacific Stratification

Since the first evidence of low algal productivity during ice ages in the Antarctic Zone of the Southern Ocean was discovered, there has been debate as to whether it was associated with increased polar ocean stratification or with sea-ice cover, shortening the productive season. The sediment concentration of biogenic barium at Ocean Drilling Program site 882 indicates low algal productivity during ice ages in the Subarctic North Pacific as well. Site 882 is located southeast of the summer sea-ice extent even during glacial maxima, ruling out sea-ice-driven light limitation and supporting stratification as the explanation, with implications for the glacial cycles of atmospheric carbon dioxide concentration.

Middle Eocene to early Oligocene paleoceanography from Agulhas Ridge, Southern Ocean (Ocean Drilling Program Leg 177, Site 1090)

The Agulhas Ridge, off the tip of Africa between the Atlantic and Indian Oceans, is ideally located to capture the evolution of Paleogene‐early Neogene circulation patterns associated with global cooling. Multiproxy records of productivity (biogenic barium (Baex), opal, CaCO3 mass accumulation rates (MARs)), nutrient and organic carbon burial (reactive phosphorus (Pr) MARs), and redox state of deep waters (U enrichment) from Ocean Drilling Program (ODP) Site 1090 reflect hydrographic shifts in this region between the middle Eocene and early Oligocene (∼39–33 Ma). Several peaks in increased export productivity and burial of organic matter occurred within the late Eocene (∼36.5, ∼34, and ∼33.7 Ma), which along with surface hydrologic conditions favoring opaline organisms over calcareous organisms could have aided in the draw down of pCO2 to a threshold level that facilitated large ice sheet development on Antarctica in the earliest Oligocene. Our multiproxy approach illustrates the importance of vertical as well as spatial hydrographic reorganization in amplifying or driving climatic cooling of the middle Eocene to early Oligocene by facilitating increases in the relative or absolute burial of organic carbon.

Particulate barium fluxes on the continental margin: a study from the Alboran Sea (Western Mediterranean)

Particulate biogenic barium (bio-Ba) fluxes obtained from three instrumented arrays moored in the Alboran Sea, the westernmost basin in the Mediterranean Sea, are presented in this study. The mooring lines were deployed over almost 1 year, from July 1997 to May 1998, and were equipped with sediment traps at ∼500–700 m depth, ∼1000–1200 m depth and 30 m above the seafloor (∼1000–2200 m). The results obtained support the growing body of evidence that the relationship between particulate bio-Ba and Corg throughout the water column in margin systems is clearly different from this relation in the open ocean. In the Alboran Sea, the annual averaged bio-Ba fluxes range from 0.39 to 1.07 μmol m −2 day −1, with mean concentrations of 1.31–1.69 μmol g −1 and bio-Ba/Corg ratios lower than in the open ocean. The low bio-Ba values obtained also indicate that calculating bio-Ba is extremely sensitive to the detrital Ba/Al ratio of each sample. The lithogenic Ba fraction in the Alboran Sea continental margin area contributes between 24% and 85% of the total Ba. Increased bio-Ba export efficiency was observed after periods of high primary productivity and suggests that the processes limiting the bio-Ba formation in the study area relate to settling dynamics of organic matter aggregates. Furthermore, the ballasting effect of the abundant lithogenic and carbonate particles may limit decomposition of organic matter aggregates and enhance the transfer of particles rich in Corg and relatively poor in bio-Ba to the deep seafloor. Lateral input of freshly sedimented biogenic material, including particulate bio-Ba, has been observed on the lower continental slope in the western Alboran Sea. These observations emphasize that the use of the bio-Ba as a proxy of export productivity from the surface ocean must be used cautiously in highly dynamic environments such as those in the Alboran Sea.

Biogenic barium and the detrital Ba/Al ratio: a comparison of their direct and indirect determination

Biogenic barium concentrations obtained from direct determination by a three-step sequential extraction procedure are compared to those obtained from the widely used indirect normative calculation based on total digestion. A comparison of the biogenic barium from the direct/sequential extraction and the indirect/normative calculation clearly shows that the detrital Ba/Al ratio is the critical factor in the normative approach, and that erroneous assumptions based on this ratio may introduce significant errors to the calculated biogenic barium. Overall, the crustal average Ba/Al ratio of 0.005–0.01 is much higher than our directly determined global average of 0.0037. This would result in an underestimation of the biogenic barium and thus of the primary productivity calculated using the Ba-flux obtained from the normatively calculated Ba record. Using our (Ba/Al) det ratio of ∼0.0037 leads to normatively calculated biogenic barium results that are in reasonable agreement with the biogenic barium from sequential extraction for samples of the Atlantic, Pacific and Indian Ocean. Our directly determined ‘regional’ (Ba/Al) det ratios deviate from those calculated or assumed from hinterland ratios and our global average (Ba/Al) det is lower than the one commonly reported. Therefore, in sediments with a significant terrigenous fraction, the sequential extraction technique is always required.

Paleoproductivity, nutrient burial, climate change and the carbon cycle in the western equatorial Atlantic across the Eocene/Oligocene boundary

Paleoproductivity, nutrient burial, and carbon cycling were investigated across the Eocene/Oligocene (E/O) boundary (begin to end; 36.9–32.7 Ma at ∼40 kyr resolution, timescale of Shackleton et al. [1999]) at Ocean Drilling Program Site 925 on the Ceara Rise in the western equatorial Atlantic (3040 m present water depth; 748.26–850.70 mbsf). Downcore bulk sediment records of biogenic barium, total reactive phosphorus, biogenic silica, and calcium carbonate are interpreted to represent export production, net nutrient burial, biogenic opal production, and inorganic carbon burial, respectively. The global positive excursion in δ13C subsequent to the E/O boundary is recorded at Site 925. Export production appears to have been externally forced by orbital parameters at eccentricity frequencies during the study interval, based on spectral analysis of the biogenic barium and reactive phosphorus records. Biogenic silica production or preservation increased after the Eocene/Oligocene boundary to a higher baseline, although overall productivity and nutrient burial did not increase, based on barium and reactive phosphorus records. Thus, although absolute production did not increase at this site, a shift in relative abundance of siliceous versus carbonate productivity may have resulted in a change in relative organic carbon burial. This may have contributed to the positive excursion in global oceanic δ13C subsequent to the Eocene/Oligocene boundary, although the silica maximum persists after the carbon isotope excursion ends.

Late Cretaceous and early Paleogene nutrient and paleoproductivity records from Blake Nose, western North Atlantic Ocean

We evaluate phosphorus (P) and biogenic barium (bio‐Ba) as nutrient burial and export productivity indicators for the Late Cretaceous and early Paleogene, combining these with calcium carbonate (CaCO3), organic carbon (C), and bulk CaCO3 C isotopes (δ13C). Sample ages span 36–71 Ma (∼1 sample/0.5 m.y.) for a depth transect of sites in the western North Atlantic (Blake Nose, Ocean Drilling Program Leg 171B, Sites 1052, 1051, and 1050). We use a multitracer approach including redox conditions to investigate export productivity surrounding the global Paleocene δ13C maximum (∼57 Ma). Reducing conditions render most of the bio‐Ba record not useful for export productivity interpretations. P and organic C records indicate that regional nutrient and organic C burial were high at ∼61 and ∼69 Ma, and low during the Paleocene δ13C maximum, a time of proposed global high relative organic C burial. Observed organic C burial changes at Blake Nose cannot explain this C isotope excursion.

Distribution of clay minerals and proxies for productivity in surface sediments of the Bellingshausen and Amundsen seas (West Antarctica) – Relation to modern environmental conditions

Surface sediments from the Antarctic continental margin in the Bellingshausen and Amundsen seas (Pacific sector of the Southern Ocean) were investigated in order to decipher their capability to record modern environmental conditions. Spatial distribution of terrigenous sand and mud reflect regional differences in current-induced redeposition of glaciogenic debris. Clay mineral assemblages in the shelf sediments are controlled by the supply of terrigenous detritus from source rocks in the adjacent hinterland suggesting the occurrence of yet unknown sedimentary rocks in the hinterland of the Amundsen Sea. Clay mineral distribution on the continental rise in the Bellingshausen Sea points to the continuation of a bottom current from the Antarctic Peninsula rise to at least 94°W. Foraminifer-bearing and opal-poor deposits prevail on the continental margin in the western Bellingshausen Sea and the Amundsen Sea, whereas diatom-bearing and carbonate-free sediments characterize the eastern Bellingshausen Sea. Different modes of biological production, which were deduced from accumulation rates of biogenic barium during Marine Isotope Stage 1 and recent productivity measurements, obviously control the spatial pattern of opal- and carbonate-bearing sediments in the study area.