Coccoliths and carbonate grain-size compositions in sediment samples from the Mid-Atlantic Ridge

This study presents a differentiated carbonate budget for marine surface sediments from the Mid-Atlantic Ridge of the South Atlantic, with results based on carbonate grain-size composition. Upon separation into sand, silt, and clay sub-fractions, the silt grain-size distribution was measured using a SediGraph 5100. We found regionally characteristic grain-size distributions with an overall minimum at 8 µm equivalent spherical diameter (ESD). SEM observations reveal that the coarse particles (>8 µm ESD) are attributed to planktic foraminifers and their fragments, and the fine particles (<8 µm ESD) to coccoliths. On the basis of this division, the regional variation of the contribution of foraminifers and coccoliths to the carbonate budget of the sediments are calculated. Foraminifer carbonate dominates the sediments in mesotropic regions whereas coccoliths contribute most carbonate in oligotrophic regions. The grain size of the coccolith share is constant over water depth, indicating a lower susceptibility for carbonate dissolution compared to foraminifers. Finally, the characteristic grain-size distribution in fine silt (<8 µm ESD) is set into context with the coccolith assemblage counted and biometrically measured using a SEM. The coccoliths present in the silt fraction are predominantly large species (length > 4 µm). Smaller species (length < 4 µm) belong to the clay fraction (<2 µm ESD). The average length of most frequent coccolith species is connected to prominent peaks in grain-size distributions (ESD) with a shape factor. The area below Gaussian distributions fitted to these peaks is suggested as a way to quantitatively estimate the carbonate contribution of single coccolith species more precisely compared to conventional volume estimates. The quantitative division of carbonate into the fraction produced by coccoliths and that secreted by foraminifers enables a more precise estimate for source/sink relations of consumed and released CO2 in the carbon cycle. The allocation of coccolith length and grain size (ESD) suggests size windows for the separation or accumulation of distinct coccolith species in investigations that depend on non to slightly-mixed signals (e.g., isotopic studies).

Alkenones and coccolithophorids in late Quaternary sediments from the Walvis Ridge

Sea surface temperatures (SSTs) derived from the alkenone UK'37) record of Quaternary sediments may be subject to bias if algae with different temperature sensitivities have contributed to the sedimentary alkenone record. The alkenone-derived SST records are usually based on a UK'37-temperature relationship which was measured in culture experiments using the coccolithophorid Emiliania huxleyi (F.G. Prahl, L.A. Muehlhausen and D.L. Zahnle, 1988. Further evaluation of long-chain alkenones as indicators of paleoceanographic conditions. Geochim. Cosmochim. Acta 52, 2303-2310). To assess possible effects of past species changes on the UK'37-temperature signal, we have analyzed long-chain alkenones and coccolithophorids in a late Quaternary sediment core from the Walvis Ridge and compared the results to SST estimates extracted from the d18O record of the planktonic foraminifer Globigerinoides ruber. Alkenones and isotopes were determined over the entire 400-kyr core record while the coccolithophorid study was confined to the last 200 kyr when the most pronounced changes in alkenone content occurred. Throughout oxygen-isotope stages 6 and 5, species of the genus Gephyrocapsa were the predominating coccolithophorids. E. huxleyi began to increase systematically in relative abundance since the stage 5/4 transition, became dominant over Gephyrocapsa spp. during stage 3 and reached the highest abundances in the Holocene. Carbon-normalized alkenone concentrations are inversely related to the relative abundances of E. huxleyi, and directly related to that of Gephyrocapsa spp., suggesting that species of this genus were the principal alkenone contributors to the sediments. Nevertheless, SST values obtained from the UK'37-temperature relationship for E. huxleyi compare favourably to the isotope-derived temperatures. The recently reported UK'37-temperature relationship for a single strain of Gephyrocapsa oceanica (J.K. Volkman. S.M. Barrett, S.I. Blackburn and E.L. Sikes, 1995. Alkenones in Gephyrocapsa oceanica: Implications for studies of paleoclimate. Geochim. Cosmochim. Acta 59, 513-520) produces unrealistically high SST values indicating that the temperature response of the examined strain is not typical for the genus Gephyrocapsa. This is supported by the C37:C38, alkenone ratios of the sediments which are comparable to average ratios reported for E. huxleyi, but significantly higher than for the G. oceanica strain. Most notably, the general accordance of the alkenone characteristics between sediments and E. huxleyi persists through stages 8 to 5 and even in times that predate the first appearance of this species (268 ka; H.R. Thierstein, K.R. Geitzenauer and B. Molfino, 1977. Global synchroneity of late Quaternary coccolith datum levels: Validation by oxygen isotopes. Geology 5, 400-404). Our results suggest that UK'37-temperature relationships based on E. huxleyi produce reasonable paleo-SST estimates even for late Quaternary periods when this species was scarce or absent because other alkenone-synthesizing algae, e.g. of the genus Gephyrocapsa.

Methane, helium isotope and hydrogen data from the Nibelungen hydrothermal area, Mid Atlantic Ridge

Hydrothermal emission of mantle helium appears to be directly related to magma production rate, but other processes can generate methane and hydrogen on mid-ocean ridges. In an on-going effort to characterize these processes in the South Atlantic, the flux and distribution of these gases were investigated in the vicinity of a powerful black smoker recently discovered at 8°17.9' S, 13°30.4' W. The vent lies on the shoulder of an oblique offset in the Mid-Atlantic Ridge and discharges high concentrations of methane and hydrogen. Measurements during expeditions in 2004 and 2006 show that the ratio of CH4 to 3He in the neutrally buoyant plume is quite high, 4 x 10**8. The CTD stations were accompanied by velocity measurements with lowered acoustic Doppler current profilers (LADCP), and from these data we estimate the methane transport to have been 0.5 mol/sec in a WSW-trending plume that seems to develop during the ebb tidal phase. This transport is an order of magnitude greater than the source of CH4 calculated from its concentration in the vent fluid and the rise height of the plume. From this range of methane fluxes, the source of 3He is estimated to be between 0.14 and 1.2 nmol/sec. In either case, the 3He source is significantly lower than expected from the spreading rate of the Mid-Atlantic Ridge. From the inventory of methane in the rift valley adjacent to the vent, it appears that the average specific rate of oxidation is 2.6 to 23/yr, corresponding to a turnover time between 140 and 16 days. Vertical profiles of methane in the surrounding region often exhibited Gaussian-like distributions, and the variances appear to increase with distance from the vent. Using a Gaussian plume model, we obtained a range of vertical eddy diffusivities between 0.009 and 0.08 m2m2/sec. These high values may be due to tidally driven internal waves across the promontory on which the vent is located.

Low-molecular-weight hydrocarbons in sediments of Broken Ridge and Ninetyeast Ridge

Core samples taken during Leg 121 drilling aboard the JOIDES Resolution in the central Indian Ocean were analyzed for their low-molecular-weight hydrocarbon contents. Forty-three samples from the Broken Ridge and 39 samples from the Ninetyeast Ridge drill sites, deep-frozen on board immediately after recovery, were studied by a dynamic headspace technique (hydrogen-stripping/thermovaporization). Light hydrocarbons (saturated and olefinic) with two to four carbon atoms, and toluene as a selected aromatic compound, were identified. Total C2-C4 saturated hydrocarbon yields vary considerably from virtually zero in a Paleogene calcareous ooze from Hole 757B to nearly 600 nanogram/gram of dry-weight sediment (parts per billion) in a Cretaceous claystone from Hole 758A. An increase of light-hydrocarbon yields with depth, and hence with sediment temperature, was observed from Hole 758A samples down to a depth of about 500 meters below seafloor. Despite extreme data scatter due to lithological changes over this depth interval, this increased yield indicates the onset of temperature-controlled hydrocarbon formation reactions. Toluene contents are also extremely variable (generally between 10 and 100 ppb) and reach more than 300 ppb in two samples of tuffaceous lithology (Sections 121-755A-17R-4 and 121-758A-48R-4). As for the saturated hydrocarbons, there was also an increase of toluene yields with increasing depth in Hole 758A.

Geochemical composition and crystallization conditions of melts from the Mid Atlantic Ridge near Ascension Island (7-11°S)

Phase equilibria simulations were performed on naturally quenched basaltic glasses to determine crystallization conditions prior to eruption of magmas at the Mid-Atlantic Ridge (MAR) east of Ascension Island (7°11°S).The results indicate that midocean ridge basalt (MORB) magmas beneath different segments of the MAR have crystallized over a wide range of pressures (100-900MPa). However, each segment seems to have a specific crystallization history. Nearly isobaric crystallization conditions (100-300MPa) were obtained for the geochemically enriched MORB magmas of the central segments, whereas normal (N)-MORB magmas of the bounding segments are characterized by polybaric crystallization conditions (200-900MPa). In addition, our results demonstrate close to anhydrous crystallization conditions of N-MORBs, whereas geochemically enriched MORBs were successfully modeled in the presence of 0.4-1wt% H2O in the parental melts.These estimates are in agreement with direct (Fourier transform IR) measurements of H2O abundances in basaltic glasses and melt inclusions for selected samples. Water contents determined in the parental melts are in the range 0.04-0.09 and 0.30-0.55 wt% H2O for depleted and enriched MORBs, respectively. Our results are in general agreement (within ±200MPa) with previous approaches used to evaluate pressure estimates in MORB. However, the determination of pre-eruptive conditions of MORBs, including temperature and water content in addition to pressure, requires the improvement of magma crystallization models to simulate liquid lines of descent in the presence of small amounts of water. KEY WORDS: MORB; Mid-Atlantic Ridge; depth of crystallization; water abundances; phase equilibria calculations; cotectic crystallization; pressure estimates; polybaric fractionation

Late Pliocene age models of Agulhas Plateau sediment cores

An ensemble of new, high-resolution records of surface ocean hydrography from the Indian-Atlantic oceanic gateway, south of Africa, demonstrates recurrent and high-amplitude salinity oscillations in the Agulhas Leakage area during the penultimate glacial-interglacial cycle. A series of millennial-scale salinification events, indicating strengthened salt leakage into the South Atlantic, appear to correlate with abrupt changes in the North Atlantic climate and Atlantic Meridional Overturning Circulation (AMOC). This interhemispheric coupling, which plausibly involved changes in the Hadley Cell and midlatitude westerlies that impacted the interocean transport at the tip of Africa, suggests that the Agulhas Leakage acted as a source of negative buoyancy for the perturbed AMOC, possibly aiding its return to full strength. Our finding points to the Indian-to-Atlantic salt transport as a potentially important modulator of the AMOC during the abrupt climate changes of the Late Pleistocene.

Geochemical data from pore water and solids from cores GS08-153-91GC12 and GS08-153-53-GC6, Arctic Mid-Ocean Ridge

Microbial communities and their associated metabolic activity in marine sediments have a profound impact on global biogeochemical cycles. Their composition and structure are attributed to geochemical and physical factors, but finding direct correlations has remained a challenge. Here we show a significant statistical relationship between variation in geochemical composition and prokaryotic community structure within deep-sea sediments. We obtained comprehensive geochemical data from two gravity cores near the hydrothermal vent field Loki's Castle at the Arctic Mid-Ocean Ridge, in the Norwegian-Greenland Sea. Geochemical properties in the rift valley sediments exhibited strong centimeter-scale stratigraphic variability. Microbial populations were profiled by pyrosequencing from 15 sediment horizons (59,364 16S rRNA gene tags), quantitatively assessed by qPCR, and phylogenetically analyzed. Although the same taxa were generally present in all samples, their relative abundances varied substantially among horizons and fluctuated between Bacteria- and Archaea-dominated communities. By independently summarizing covariance structures of the relative abundance data and geochemical data, using principal components analysis, we found a significant correlation between changes in geochemical composition and changes in community structure. Differences in organic carbon and mineralogy shaped the relative abundance of microbial taxa. We used correlations to build hypotheses about energy metabolisms, particularly of the Deep Sea Archaeal Group, specific Deltaproteobacteria, and sediment lineages of potentially anaerobic Marine Group I Archaea. We demonstrate that total prokaryotic community structure can be directly correlated to geochemistry within these sediments, thus enhancing our understanding of biogeochemical cycling and our ability to predict metabolisms of uncultured microbes in deep-sea sediments.

Multicentennial resolution proxy records from splicings of MD96-2080 and MD02-2594, Agulhas Bank Splice (ABS)

[1] Planktonic d18O and Mg/Ca-derived sea surface temperature (SST) records from the Agulhas Corridor off South Africa display a progressive increase of SST during glacial periods of the last three climatic cycles. The SST increases of up to 4°C coincide with increased abundance of subtropical planktonic foraminiferal marker species which indicates a progressive warming due to an increased influence of subtropical waters at the core sites. Mg/Ca-derived SST maximizes during glacial maxima and glacial Terminations to values about 2.5°C above full-interglacial SST. The paired planktonic d18O and Mg/Ca-derived SST records yield glacial seawater d18O anomalies of up to 0.8 per mill, indicating measurably higher surface salinities during these periods. The SST pattern along our record is markedly different from a UK'37-derived SST record at a nearby core location in the Agulhas Corridor that displays SST maxima only during glacial Terminations. Possible explanations are lateral alkenone advection by the vigorous regional ocean currents or the development of SST contrasts during glacials in association with seasonal changes of Agulhas water transports and lateral shifts of the Agulhas retroflection. The different SST reconstructions derived from UK'37 and Mg/Ca pose a significant challenge to the interpretation of the proxy records and demonstrate that the reconstruction of the Agulhas Current and interocean salt leakage is not as straightforward as previously suggested.