Geochemistry on core 202-1240A

The modern Eastern Equatorial Pacific (EEP) Ocean is a large oceanic source of carbon to the atmosphere1. Primary productivity over large areas of the EEP is limited by silicic acid and iron availability, and because of this constraint the organic carbon export to the deep ocean is unable to compensate for the outgassing of carbon dioxide that occurs through upwelling of deep waters. It has been suggested that the delivery of dust-borne iron to the glacial ocean could have increased primary productivity and enhanced deep-sea carbon export in this region, lowering atmospheric carbon dioxide concentrations during glacial periods. Such a role for the EEP is supported by higher organic carbon burial rates documented in underlying glacial sediments but lower opal accumulation rates cast doubts on the importance of the EEP as an oceanic region for significant glacial carbon dioxide drawdown. Here we present a new silicon isotope record that suggests the paradoxical decline in opal accumulation rate in the glacial EEP results from a decrease in the silicon to carbon uptake ratio of diatoms under conditions of increased iron availability from enhanced dust input. Consequently, our study supports the idea of an invigorated biological pump in this region during the last glacial period that could have contributed to glacial carbon dioxide drawdown. Additionally, using evidence from silicon and nitrogen isotope changes, we infer that, in contrast to the modern situation, the biological productivity in this region is not constrained by the availability of iron, silicon and nitrogen during the glacial period. We hypothesize that an invigorated biological carbon dioxide pump constrained perhaps only by phosphorus limitation was a more common occurrence in low-latitude areas of the glacial ocean.

Pleistocene stable isotope record of planktonic foraminifera of ODP Site 138-847

We present Pleistocene oxygen and carbon isotope records from two planktonic foraminifer species (Globigerinoides sacculifer and Neogloboquadrina dutertrei) from Ocean Drilling Program Site 847 (0°16'N, 95°19'W; 3334 m water depth). An average sample resolution of 4500 yr was obtained by sampling at an interval of 15 cm through a continuous 35-m section from 0 to 1.15 Ma. Our d18O-based chronology is similar to that derived independently by astronomically tuning the gamma-ray attenuation porosity evaluator (GRAPE) record (Shackleton et al., 1995), though offsets as large as ± 30 k.y. occur on occasion. The surface waters at eastern equatorial Pacific Site 847, 380 km west of the Galapagos, are characterized by strong and constant upwelling, elevated nutrient concentrations, and high productivity. The isotopic composition of G. sacculifer (300-355 µm) reflects conditions in the thin-surface mixed layer, and the composition of N. dutertrei (355-425 µm) monitors the subsurface waters of the permanent shallow (10-40 m) thermocline. The Pleistocene d18O difference (N. dutertrei minus G. sacculifer, Dd18Od-s) averages 0.9 per mil and ranges from 0 per mil to 1.7 per mil. Neglecting species effects and shell size, the average Pleistocene d13C difference (G. sacculifer minus N. dutertrei, Dd13Cs-d) is 0.0 per mil and ranges from -0.5 per mil to 0.5 per mil. The Dd18Od-s and Dd13Cs-d records are used to infer vertical contrasts in upper ocean water temperature and nutrient concentration, though d13C may also be influenced by other factors, such as CO2 gas exchange. Variations in the isotopic differences are often synchronous with glacial/interglacial climate change. Glacial periods are characterized by smaller vertical contrasts in both temperature and nutrient concentration, and by notably greater accumulation rates of N. dutertrei and CaCO3. We attribute these responses to greater upwelling at the equatorial divergence. Superimposed on the glacial/interglacial Dd18Od-s pattern is a long-term trend possibly associated with the advection of Peru Current waters. The temporal fluctuations in the isotopic contrasts are strikingly similar to those observed at Site 851 (Ravelo and Shackleton, this volume), suggesting that the inferred changes in thermal and chemical profiles occurred over a broad region in the equatorial Pacific.

Sedimentation rate, bulk density, accumulation rate of terrigenous material and grain sizes of ODP Leg 167 sites

The terrigenous mineral fraction of sediments recovered by drilling during Ocean Drilling Program Leg 167 at Sites 1018 and 1020 is used to evaluate changes in the source and transport of fine-grained terrigenous sediment and its relation to regional climates and the paleoceanographic evolution of the California Current system during the late Pleistocene. Preliminary time scales developed by correlation of oxygen isotope stratigraphies with the global SPECMAP record show average linear sedimentation rates in excess of 100 m/m.y., which provide an opportunity for high-resolution studies of terrigenous flux, grain size, and mineralogy. The mass flux of terrigenous minerals at Site 1018 varies from 5 to 30 g/(cm**2 x k.y.) and displays a general trend toward increased flux during glacials. The terrigenous record at Site 1020 shows a similar pattern of increased glacial input, but overall accumulation rates are significantly lower. Spectral analysis demonstrates that most of this variability is concentrated in frequency bands related to orbital cycles of eccentricity, tilt, and precession. Detailed grain-size analysis performed on the isolated terrigenous mineral fraction shows that sediments from Site 1018 are associated with higher energy transport and depositional regimes than those found at Site 1020. Grain-size data are remarkably uniform throughout the last 500 k.y., with no discernible difference observed between glacial and interglacial size distributions within each site. X-ray diffraction analysis of the <2-µm clay component suggests that the deposition of minerals found at Site 1020 is consistent with transport from a southern source during intervals of increased terrigenous input.

Organic geochemistry of Upper Albian sediments from the Kirchrode I borehole

In the framework of a multidisciplinary research program, an organic geochemical study was carried out on a drill core which comprises a 245 m thick sequence of light-colored, Upper Albian marlstones that were deposited in the central part of the Lower Saxony basin (northern Germany). For part of the Upper Albian sequence, high-resolution measurements of carbonate contents reveal cycles which can be related to earth orbital forcing. Based on these data, sediment accumulation rates were calculated to be in the order of 15 g/m**2/yr. These high accumulation rates contrast with very low organic carbon contents and an extremely poor preservation of the autochthonous organic matter. Most of the sedimentary organic matter is of terrigenous origin and mainly derived from the erosion of older sedimentary rocks. Organic petrography reveals only a very small fraction of marine organic particles. Carbon/sulphur ratios, pristane/phytane ratios as well as the predominance of resedimented organic particles over autochthonous organic particles suggest that aerobic degradation processes rather than anaerobic processes (sulphate reduction) were responsible for the degradation of the organic matter. Furthermore, the scarcity of terrigenous organic particles (vitrinite) indicates that there was little vegetation on nearby land areas. To explain these analytical results, a depositional model was developed which could explain the scarcity of organic matter in the Upper Albian sediments. This model is based on downwelling of oxygen-rich, saline waters of Tethyan origin, which reduces the nutrient content of surface waters and thus primary bioproductivity while degradation of primary organic matter in the water column is enhanced at the same time. These conditions contrast to those which existed in Barremian and early Aptian times in this basin, when limited water exchange with adjacent oceans caused oxygen deficiency and the deposition of numerous organic carbon-rich black shales. The thick, organic matter-poor Upper Albian sequence of northern Germany also contrasts with comparatively thin, time-equivalent, deep-sea black shales from Italy. This discrepancy indicates that local and regional oceanographic factors (at least in this case) have a greater influence on organic matter deposition than global events.

Sedimentology of ODP sites in the Cape Basin, southeast Atlantic Ocean

Middle/late Miocene to early Pliocene sedimentary sequences along the continental margin of southwest Africa have changes that correspond to the carbonate crash (12-9 Ma) and biogenic bloom events (~7-4 Ma) described in the equatorial Pacific by Farrell et al. (1995, doi:10.2973/odp.proc.sr.138.143.1995). To explore the origins of these changes, we analyzed the carbon and coarse fraction contents of sediments from ODP Sites 1085, 1086, and 1087 at a time resolution of 5 to 30 kyr. Several major drops in CaCO3 concentration between 12 and 9 Ma are caused by dilution from major increases in clastic input from the Oranje River during global sea level regressions. Abundant pyrite crystals and good preservation of fish debris reflect low oxygenation of bottom/pore waters. Regional productivity was enhanced during the time equivalent to the carbonate crash period. Higher benthic/planktic foraminiferal ratios indicate that CaCO3 dissolution at Site 1085 peaked between 9 to 7 Ma, which was after the global carbonate crash. This period of enhanced dissolution suggests that Site 1085 was located within a low-oxygen water mass that dissolved CaCO3 more easily than North Atlantic Deep Water, which began to bathe this site at 7 Ma. At 7 to 6 Ma, the onset of the biogenic bloom, increases and variations in total organic carbon and benthic foraminiferal accumulation rates show that paleoproductivity increased significantly above values observed during the carbonate crash period and fluctuated widely. We attribute the late Miocene paleoproductivity increase off southwest Africa to ocean-wide increases in nutrient supply and delivery.

Stable isotopic record of late Pliocene and Pleistocene foraminifera of ODP Site 114-704

We studied the stable isotopic and carbonate stratigraphy of ODP Hole 704A to reconstruct the paleoceanographic evolution of the eastern subantarctic sector of the South Atlantic Ocean. Site 704 is well positioned with respect to latitude (46°52.8'S, 7°25.3'E) and bathymetry (2532 m) to monitor past migrations in the position of Polar Front Zone (PFZ) and changes in deep-water circulation during the late Pliocene-Pleistocene. Several important changes occurred in proxy paleoceanographic indicators across the Gauss/Matuyama boundary at 2.47 Ma: (1) accumulation rates of biogenic sedimentary components increased by an order of magnitude (Froelich et al., this volume); (2) planktonic d1 8O values increased by an average of 0.5 per mil; (3) the amplitude of the benthic d18O signal increased; (4) the accumulation rate of ice-rafted detritus increased several fold (Warnke and Allen, this volume); and (5) carbon isotopic ratios of benthic foraminifers decreased by 0.5 per mil, as did the d13C of the fine-fraction carbonate by 1.5 per mil (Mead et al., 1991, doi:10.2973/odp.proc.sr.114.152.1991), but no change occurred in planktonic foraminiferal d13C values. Most of these changes are consistent with more frequent expansions and contractions of the PFZ over Site 704 after 2.47 Ma, bringing cold, nutrient-rich waters to 47°S that stimulated both carbonate and siliceous productivity. The synchronous increase in d18O values and ice-rafted detritus accumulation in Hole 704A indicates that the 2.4 Ma paleoceanographic event included ice volume growth on both Antarctica and Northern Hemisphere continents. The decrease in benthic d13C values indicates that the ventilation rate of Southern Ocean deep water decreased and the nutrient content increased during glacial events after 2.5 Ma. At the Gauss/Matuyama boundary, benthic d13C values of the Southern Ocean shifted toward those of the Pacific end member, indicating a decrease in the relative mixing ratio of Northern Component Water and Circumpolar Deep Water. During the early Matuyama (~2.3 to 1.7 Ma), the PFZ generally occupied a southerly position with respect to Site 704 and carbonate productivity prevailed. Exceptions to these general conditions occurred during strong glacial events of the early Matuyama (e.g., isotopic stages 82, 78, 74, and 70), when the PFZ migrated to the north and opal sedimentation predominated at Site 704. At 1.7 Ma, the PFZ migrated toward the equator and occupied a more northerly position for a prolonged interval between ~1.7 and 1.5 Ma. Beginning at ~1.5-1.4 Ma, surface and bottom water parameters (d18O, d13C, %CaCO3, and %opal) in the subantarctic South Atlantic became highly correlated such that glacial events (d18O maxima) corresponded to d13C and carbonate minima and opal maxima. This pattern is typical of the correlation found during the latest Pleistocene in the Southern Ocean (Charles and Fairbanks, in press). This event coincided with increased suppression of Northern Component Water during glacial events after 1.5 Ma (Raymo et al., 1990, doi:10.1016/0012-821X(90)90051-X), which may have influenced the climatology of the Southern Hemisphere by altering the flux of heat and salt to the Southern Ocean).

Abundance of organic-walled dinoflagellate cysts and geochemical analysis of sediment core GeoB5546-2

NW African climate shows orbital and millenial-scale variations, which are tightly connected to changes in marine productivity. We present an organic-walled dinoflagellate cyst (dinocyst) record from a sediment core off Cape Yubi at about 27°N in the Canary Basin covering the time period from 47 to 3ka before present (BP). The dinocyst record reflects differences in upwelling intensity and seasonality as well as the influence of fluvial input. Sea-level changes play an important role for the upwelling pattern and productivity signals at the core site. Within the studied time interval, four main phases were distinguished. (1) From 45 to 24ka BP, when sea-level was mostly about 75m lower than today, high relative abundances of cysts of heterotrophic taxa point to enhanced upwelling activity, especially during Heinrich Events, while relatively low dinocyst accumulation rates indicate that filament activity at the core location was strongly reduced. (2) At sea-level lowstand during the LGM to H1, dinocyst accumulation rates suggest that local filament formation was even more inhibited. (3) From the early Holocene to about 8ka BP, extraordinary high accumulation rates of most dinocyst species, especially of Lingulodinium machaerophorum, suggest that nutrient supply via fluvial input increased and rising sea-level promoted filament formation. At the same time, the upwelling season prolongated. (4) A relative increase in cysts of photoautotrophic taxa from about 8ka BP on indicates more stratified conditions while fluvial input decreased. Our study shows that productivity records can be very sensitive to regional features. From the dinocyst data we infer that marine surface productivity off Cape Yubi during glacial times was within the scale of modern times but extremely enhanced during deglaciation.

Benthic foraminifera in Tertiary sediments of DSDP Leg 90 holes

Eocene through Pliocene benthic foraminifers were examined from seven sites located at middle and lower bathyal depths on the Lord Howe Rise in the Tasman Sea, from another site at lower bathyal depths in the Coral Sea, and from a site in the intermediate-depth, hemipelagic province of the Chatham Rise, east of southern New Zealand. Age-related, depth-related, and bioprovincial faunal variations are documented in this chapter. One new species, Rectuvigerina tasmana, is named. The paleoecologic indications of several key groups, including the miliolids, uvigerinids, nuttallitids, and cibicidids, are combined with sedimentologic and stable isotopic tracers to interpret paleoceanographic changes in the Tasman Sea. Because the total stratigraphic ranges of many bathyal benthic foraminifers are not yet known, most endpoints in the Tasman Sea are considered ecologically controlled events. The disappearances of Uvigerina rippensis and Cibicidoidesparki and the first appearances of U. pigmaea, Sphaeroidina bulloides, and Rotaliatina sulcigera at the Eocene/Oligocene boundary can be considered evolutionary events, as also can the first appearance of Cibicides wuellerstorfi in Zone NN5. Species which are restricted to the lower bathyal zone except during discrete pulses, most of which are related to the development of glacial conditions, include Melonis pompilioides, M. sphaeroides, Pullenia quinqueloba, Nuttallides umbonifera, and U. hispido-costata. Middle bathyal indigenes include U. spinulosa, U. gemmaeformis, Ehrenbergina marwicki, R. sulcigera, and all rectuvigerinids except Rectuvigerina spinea. Although the miliolids first occurred at lower bathyal depths, they were more common in the middle bathyal zone. Although the Neogene hispido-costate uvigerinids first developed at lower bathyal depths and at higher middle latitude sites, in the later Neogene this group migrated to shallower depths and became predominant also in the middle bathyal zone. Despite the relatively similar sedimentologic settings at the six middle bathyal Tasman sites, there was extensive intrageneric and intraspecific geographic variation. Mililiolids, strongly ornamented brizalinids, bolivinitids, Bulimina aculeata, Osangularia culter, and strongly porous morphotypes were more common at higher latitudes. Osangularia bengalensis, striate brizalinids such as Brizalina subaenariensis, Gaudryina solida, osangularids in general, and finely porous morphotypes were more common in the subtropics. There was strong covariance between faunas at lower middle latitude, lower bathyal Site 591, and higher middle latitude, middle bathyal Site 593. The following oceanographic history of the Tasman Sea is proposed; using the stable isotopic record as evidence for glacials and examining the ecologic correlations between (1) miliolids and carbonate saturation, (2) nuttallitids and undersaturated, cooled, or "new" water masses, (3) uvigerinids with high organic carbon in the sediment and high rates of sediment accumulation, and (4) cibicidids and terrestrial organic carbon. The glacial located near the Eocene/Oligocene boundary is characterized by the penetration of cooler, more corrosive waters at intermediate depths in high southern latitudes. This may have caused overturn, upwelling pulses, in other Tasman areas. The development of Neogenelike conditions began in the late Oligocene (Zone NP24/NP25) with the evolution of several common Neogene species. A large number of Paleogene benthics disappeared gradually through the course of the early Miocene, which was not well preserved at any Tasman site. Corrosive conditions shallowed into the middle bathyal zone in several pulses during the early Miocene. The development of glacial conditions in the middle Miocene was accompanied by major changes throughout the Tasman Sea. Sediment accumulation rates increased and high-productivity faunas and corrosive conditions developed at all but the lowest-latitude Site 588. This increase in productivity and accumulation rate is attributed to the eutrophication of Antarctic water masses feeding Tasman current systems, as well as to invigorated circulation in general. It overlaps with the beginning of the Pacific High-productivity Episode (10-5 Ma). During the latest Miocene glacial episode, corrosive conditions developed at lower bathyal depths, while cooler water and lower nutrient levels shallowed to middle bathyal depths. Lower input of terrestrial organic carbon may be related to the lower nutrient levels of this time and to the termination of the Pacific High-productivity Episode. The moderate glacial episode during the mid-Pliocene (Zone NN15/NN16, ~3.2 Ma) corresponds to a decline in sediment accumulation rates and a reorganization of faunas unlike that of all other times. New genera proliferate and indices for cool, noncorrosive conditions and high organic carbon expand throughout the middle bathyal zone coeval with the sedimentation rate decreases. By the latest Pliocene (about 2.5 Ma), however, during another glacial episode, faunal patterns typical of this and later glacials develop throughout the Tasman Sea. Benthic foraminiferal patterns suggest increased input of terrestrial organic matter to Tasman Sea sediments during this episode and during later glacials.

Age determination of sediment cores from the Northeast Atlantic

Radiocarbon dating series, bulk sediment, and organic carbon flux from various Atlantic deep-sea regions reveal that the thickness of the bioturbated zone increases by 2 cm if food supply increases by 1 gC/m**2/yr (r = 0.8). Bulk sediment accumulation rates do not influence the depth of bioturbational mixing under normal pelagic sedimentary conditions. We believe that this relationship between nutrient supply and benthic mixing can be used for a quantitative and time-variable unmixing procedure to improve high-resolution stratigraphic correlations and paleoclimatic interpretations of deep-sea records.

Seawater carbonate chemistry and nutrient fluxes during experiments with brittlestar Amphiura filiformis, 2009

Rising levels of atmospheric carbon dioxide and the concomitant increased uptake of this by the oceans is resulting in hypercapnia-related reduction of ocean pH. Research focussed on the direct effects of these physicochemical changes on marine invertebrates has begun to improve our understanding of impacts at the level of individual physiologies. However, CO2-related impairment of organisms' contribution to ecological or ecosystem processes has barely been addressed. The burrowing ophiuroid Amphiura filiformis, which has a physiology that makes it susceptible to reduced pH, plays a key role in sediment nutrient cycling by mixing and irrigating the sediment, a process known as bioturbation. Here we investigate the role of A. filiformis in modifying nutrient flux rates across the sediment-water boundary and the impact of CO2- related acidification on this process. A 40 day exposure study was conducted under predicted pH scenarios from the years 2100 (pH 7.7) and 2300 (pH 7.3), plus an additional treatment of pH 6.8. This study demonstrated strong relationships between A. filiformis density and cycling of some nutrients; activity increases the sediment uptake of phosphate and the release of nitrite and nitrate. No relationship between A. filiformis density and the flux of ammonium or silicate were observed. Results also indicated that, within the timescale of this experiment, effects at the individual bioturbator level appear not to translate into reduced ecosystem influence. However, long term survival of key bioturbating species is far from assured and changes in both bioturbation and microbial processes could alter key biogeochemical processes in future, more acidic oceans.

Age models and stable isotope ratios of SO12 and SO35 sediment cores

Based on organic carbon accumulation rates, nine time slices of oceanic export paleoproductivity (Pnew) are presented which depict the variability of Pnew on a global scale through the last 30,000 years and document that the basic distribution patterns did not change through glacial and interglacial times. However, the glacial ocean shows an increased contrast of high- versus low-productivity zones. d13C values of near-surface-dwelling planktonic foraminifera Globigerinoides ruber suggest that the same contrast applies to the glacial nutrient inventories of the ambient surface waters, with a significant glacial transfer of PO4 from low- to high-productivity zones. In this way, glacial Pnew increased by a global average of about 2-4 Gt C/yr and led, via an enhanced CaCO3 dissolution and alkalinity in the deep ocean, to a significant extraction of CO2 from the surface water and the atrnosphere.