Phosphorus components, CaCO3 and biogenic opal in sediments from ODP Leg 167 sites

The ocean history of reactive phosphorus (P) (i.e., dissolved P available to fuel oceanic primary productivity) is of interest because of the role of P as a biolimiting nutrient, and knowledge of P burial in marine sediments is key to testing hypotheses about temporal changes in P input or output fluxes. Our understanding of the history of the P cycle over the Cenozoic has increased substantially with temporal records of reactive P mass accumulation rates from open-ocean Pacific and Atlantic equatorial sites. However, questions about the relative importance of nutrient burial in ocean-margin sediments relative to burial in open-ocean sediments and about the extent of P remobilization in organic-rich, reducing environments characteristic of margin sediments remain unresolved. Nutrient burial in oceanic boundary current systems has been suggested to have a controlling role in oceanic nutrient budgets in certain time intervals (Vincent and Berger, 1985, doi:10.1029/GM032p0455), with higher sediment accumulation rates balancing the limited spatial extent of these sediments. Some investigators suggest that remobilization of P from reducing sediments in margin settings is a significant positive feedback to primary productivity (e.g., Van Cappellan and Ingall, 1994, doi:10.1029/94PA01455), whereas other results indicate that both P uptake and P release may occur in these settings depending on the balance of organic carbon and iron supply to the sediments and on the oxygenation of bottom waters (McManus et al., 1997, doi:10.1016/S0016-7037(97)00138-5). It is important to quantitatively understand the geochemistry of reactive P in margin sediments, where productivity and delivery of organic-rich material to the sediments in relatively shallow-water settings is often sufficient to promote anoxia in interstitial waters. To address these questions, we determined the P concentrations and geochemistry in sediment samples from eight sites drilled during Ocean Drilling Program (ODP) Leg 167, California margin (Sites 1010-1012, 1014, 1016-1017, and 1021-1022). These results are the first records of reactive P concentrations on long time scales-required for the calculation of P accumulation rates-for sediments from a highly productive eastern boundary current setting. In addition, we determined calcium carbonate contents and biogenic silica concentrations to define the environments of sedimentary production, burial, and diagenesis.

Sulphur contents and partition of iron in ODP Leg 112 sediments

Porewaters in site 680 Peru Margin sediments contain dissolved sulfide over a depth of approximately 70 m which, at a sedimentation rate of 0.005 cm/yr, gives a sediment exposure time to dissolved sulfide of about 1.4 Myr. Reactions with dissolved sulfide cause the site 680 sediments to show a progressive decrease in a poorly-reactive silicate iron fraction, defined as the difference between iron extracted by dithionite (FeD) at room temperature and that extracted by boiling concentrated HCl (FeH), normalised to the total iron content (FeT). Straight line plots are obtained for ln[(FeH - FeD)/FeT] against time of burial, from which a first order rate constant of 0.29 1/Myr (equivalent to a half-life of 2.4 Myr) can be derived for the sulfidation of this silicate iron. Comparable half-lives are also found for the same poorly-reactive iron fraction in the nearby site 681 and 684 sediments. This silicate Fe fraction comprises 0.8-1.0% Fe, only 30-60% of which reacts even with 1.5-3 million years exposure to dissolved sulfide. Diagenetic models based on porewater concentrations of sulfate and sulfide, and solid phase iron contents, at site 680 are consistent in indicating that this poorly-reactive iron fraction is only sulfidized on a million year time scale. Silicate iron not extracted by HCl can be regarded as unreactive towards dissolved sulfide on the time scales encountered in marine sediments.

(Table 1) Micro-inclusion counts and ion concentrations in the five GRIP ice-core sections

We provide the first direct evidence that a number of water-soluble compounds, in particular calcium sulfate (CaSO4 2H2O) and calcium carbonate (CaCO3), are present as solid, micron-sized inclusions within the Greenland GRIP ice core. The compounds are detected by two independent methods: micro-Raman spectroscopy of a solid ice sample, and energy-dispersive X-ray spectroscopy of individual inclusions remaining after sublimation. CaSO4 2H2O is found in abundance throughout the Holocene and the last glacial period, while CaCO3 exists mainly in the glacial period ice. We also present size and spatial distributions of the micro-inclusions. These results suggest that water-soluble aerosols in the GRIP ice core are dependable proxies for past atmospheric conditions.

Chemical composition of glass inclusions from ODP Holes 157-953C and 157-956B

We report S concentrations and relative proportions of (SO4)2- and S2- in OL- and CPX-hosted glass inclusions and in host glassy lapilli from Miocene basaltic hyaloclastites drilled north and south of Gran Canaria during ODP Leg 157. Compositions of glass inclusions and lapilli resemble those of subaerial Miocene shield basalts on Gran Canaria and comprise mafic to more evolved tholeiitic to alkali basalt and basanite (10.3-3.7 wt.% MgO, 44.5-56.9 wt.% SiO2). Glass inclusions fall into three groups based on their S concentrations: a high-sulfur group (1050 to 5810 ppm S), an intermediate-sulfur group (510 to 1740 ppm S), and a low-sulfur group (<500 ppm S). The most S-rich inclusions have the highest and nearly constant proportion of sulfur dissolved as sulfate determined by electron microprobe measurements of SKa peak shift. Their average S6+/S_total value is 0.75+/-0.09, unusually high for ocean island basalt magmas. The low-sulfur group inclusions have low S6+/S_total ratios (0.08+/-0.05), whereas intermediate sulfur group inclusions show a wide range of S6+/S_total (0.05-0.83). Glassy lapilli and their crystal-hosted glass inclusions with S concentrations of 50 to 1140 ppm S have very similar S6+/S_total ratios of 0.36+/-0.06 implying that sulfur degassing does not affect the proportion of (SO4)2- and S2- in the magma. The oxygen fugacities estimated from S6+/S_total ratios and from Fe3+/Fe2+ ratios in spinel inclusions range from NNO-1.1 to NNO+1.8. The origin of S-rich magmas is unclear. We discuss (1) partial melting of a mantle source at relatively oxidized fO2 conditions, and (2) magma contamination by seawater either directly or through magma interaction with seawater-altered Jurassic oceanic crust. The intermediate sulfur group inclusions represent undegassed or slightly degassed magmas similar to submarine OIB glasses, whereas the low-sulfur group inclusions are likely to have formed from magmas significantly degassed in near-surface reservoirs. Mixing of these degassed magmas with stored volatile-rich ones or volatile-rich magma replenishing the chamber filled by partially degassed magmas may produce hybrid melts with strongly varying S concentrations and S6+/S_total ratios.

Composition of matrix glasses and melt inclusions of fallout tephra from ODP Hole 125-782A

We studied the systematics of Cl, F and H2O in Izu arc front volcanic rocks using basaltic through rhyolitic glass shards and melt inclusions (Izu glasses) from Oligocene to Quaternary distal fallout tephra. These glasses are low-K basalts to rhyolites that are equivalent to the Quaternary lavas of the Izu arc front (Izu VF). Most of the Izu glasses have Cl ~400-4000 ppm and F ~70-400 ppm (normal-group glasses). Rare andesitic melt inclusions (halogen-rich andesites; HRA) have very high abundances of Cl (~6600-8600 ppm) and F (~780-910 ppm), but their contents of incompatible large ion lithophile elements (LILE) are similar to the normal-group glasses. The preeruptive H2O of basalt to andesite melt inclusions in plagioclase is estimated to range from ~2 to ~10 wt% H2O. The Izu magmas should be undersaturated in H2O and the halogens at their preferred levels of crystallization in the middle to lower crust (~3 to ~11 kbar, ~820° to ~1200°C). A substantial portion of the original H2O is lost due to degassing during the final ascent to surface. By contrast, halogen loss is minor, except for loss of Cl from siliceous dacitic and rhyolitic compositions. The behavior of Cl, F and H2O in undegassed melts resembles the fluid mobile LILE (e.g.; K, Rb, Cs, Ba, U, Pb, Li). Most of the Cl (>99%), H2O (>95%) and F (>53%) in the Izu VF melts appear to originate from the subducting slab. At arc front depths, the slab fluid contains Cl = 0.94+/-0.25 wt%, F = 990+/-270 ppm and H2O = 25+/-7 wt%. If the subducting sediment and the altered basaltic crust were the only slab sources, then the subducted Cl appears to be almost entirely recycled at the Izu arc (~77-129%). Conversely, H2O (~13-22% recycled at arc) and F (~4-6% recycled) must be either lost during shallow subduction or retained in the slab to greater depths. If a seawater-impregnated serpentinite layer below the basaltic crust were an additional source of Cl and H2O, the calculated percentage of Cl and H2O recycled at arc would be lower. Extrapolating the Izu data to the total length of global arcs (~37000 km), the global arc outflux of fluid-recycled Cl and H2O at subduction zones amounts to Cl ~2.9-3.8 mln ton/yr and H2O ~70-100 mln ton/yr, respectively - comparable to previous estimates. Further, we obtain a first estimate of global arc outflux of fluid-recycled F of ~0.3-0.4 mln ton/yr. Despite the inherent uncertainties, our results support models suggesting that the slab becomes strongly depleted in Cl and H2O in subduction zones. In contrast, much of the subducted F appears to be returned to the deep mantle, implying efficient fractionation of Cl and H2O from F during the subduction process. However, if slab devolatilization produces slab fluids with high Cl/F (~9.5), slab melting will still produce components with low Cl/F ratios (~0.9), similar to those characteristic of the upper continental crust (Cl/F ~0.3-0.9).

Median grain size in ODP Sites 184-1143 and 184-1144

Monsoon climate is an important component of the global climatic system. A comprehensive understanding of its variability over glacial-interglacial time scales as well as of its effects on the continent and in the ocean is required to decipher links between climate, continental weathering and productivity. A detailed multiproxy study, including bulk and clay mineralogy, grain-size analysis, phosphorus geochemistry (SEDEX extraction), organic matter characterization, and nitrogen stable isotopes, was carried out on samples from ODP Sites 1143 and 1144 (Leg 184, South China Sea), covering the past 140 000 years. We tentatively reconstruct the complex sedimentation and climatic history of the region during the last glacial-interglacial cycle, when sea-level variations, linked to the growth and melting of ice caps, interact with monsoon variability. During interglacial periods of high sea level, summer monsoon was strong, and humid and warm climate characterized the adjacent continent and islands. Clay minerals bear signals of chemical weathering during these intervals. High calcite and reactive phosphorus mass accumulation rates (MARs) indicate high productivity, especially in the southern region of the basin. During glacial intervals, strong winter monsoon provided enhanced detrital input from the continent, as indicated by high detrital MAR. Glacial low sea level resulted in erosion of sediments from the exposed Sunda shelf to the south, and clay mineral variations indicate that warm and humid conditions still prevailed in the southern tropical areas. Enhanced supply of nutrients from the continent, both by river and eolian input, maintained high primary productivity. Reduced circulation during these periods possibly induced active remobilization of nutrients, such as phosphorus, from the sediments. Intense and short cold periods recorded during glacial and interglacial stages correlate with loess records in China and marine climatic records in the North Atlantic, confirming a teleconnection between low- and high-latitude climate variability.

Shifts in the microbial community structure in different temperatures during sample storage from IODP Hole 325-M0058A

The objective of this study was to determine shifts in the microbial community structure and potential function based on standard Integrated Ocean Drilling Program (IODP) storage procedures for sediment cores. Standard long-term storage protocols maintain sediment temperature at 4°C for mineralogy, geochemical, and/or geotechnical analysis whereas standard microbiological sampling immediately preserves sediments at -80°C. Storage at 4°C does not take into account populations may remain active over geologic time scales at temperatures similar to storage conditions. Identification of active populations within the stored core would suggest geochemical and geophysical conditions within the core change over time. To test this potential, the metabolically active fraction of the total microbial community was characterized from IODP Expedition 325 Great Barrier Reef sediment cores prior to and following a 3-month storage period. Total RNA was extracted from complementary 2, 20, and 40 m below sea floor sediment samples, reverse transcribed to complementary DNA and then sequenced using 454 FLX sequencing technology, yielding over 14,800 sequences from the six samples. Interestingly, 97.3% of the sequences detected were associated with lineages that changed in detection frequency during the storage period including key biogeochemically relevant lineages associated with nitrogen, iron, and sulfur cycling. These lineages have the potential to permanently alter the physical and chemical characteristics of the sediment promoting misleading conclusions about the in situ biogeochemical environment. In addition, the detection of new lineages after storage increases the potential for a wider range of viable lineages within the subsurface that may be underestimated during standard community characterizations.

Retreat characteristics and geometries for 59 glaciers in West Greenland between 2000-2009

Recent changes in the dynamics of Greenland's marine terminating outlet glaciers indicate a rapid and complex response to external forcing. Despite observed ice front retreat and recent geophysical evidence for accelerated mass loss along Greenland's northwestern margin, it is unclear whether west Greenland glaciers have undergone the synchronous speed-up and subsequent slow-down as observed in southeastern glaciers earlier in the decade. To investigate changes in west Greenland outlet glacier dynamics and the potential controls behind their behavior, we derive time series of front position, surface elevation, and surface slope for 59 marine terminating outlet glaciers and surface speeds for select glaciers in west Greenland from 2000 to 2009. Using these data, we look for relationships between retreat, thinning, acceleration, and geometric parameters to determine the first-order controls on glacier behavior. Our data indicate that changes in front positions and surface elevations were asynchronous on annual time scales, though nearly all glaciers retreated and thinned over the decade. We found no direct relationship between retreat, acceleration, and external forcing applicable to the entire region. In regard to geometry, we found that, following retreat, (1) glaciers with grounded termini experienced more pronounced changes in dynamics than those with floating termini and (2) thinning rates declined more quickly for glaciers with steeper slopes. Overall, glacier geometry should influence outlet glacier dynamics via stress redistribution following perturbations at the front, but our data indicate that the relative importance of geometry as a control of glacier behavior is highly variable throughout west Greenland.

Lead 210 concentrations in sediments of the Arabian Sea

In order to evaluate bioturbation in abyssal Arabian-Sea sediments of the Indus fan profiles of 210Pb (half-life: 22.3 yr) and 234Th (half-life: 24.1 d) were measured in cores collected during September and October 1995 and April 1997, respectively. The density and composition of epibenthic megafauna and lebensspuren were determined in vertical seafloor photographs during April 1997. Mean eddy-diffusive mixing coefficients according to the distribution of excess 210Pb ( 210Pb-DB) were 0.072±0.028, 0.068±0.055, 0.373±0.119, 0.037±0.009 and 0.079±0.119 cm**2 yr**-1 in the northern, western, central, eastern and southern abyssal Arabian sea, respectively. Mean eddy-diffusive mixing coefficients according to the distribution of excess 234Th (234Th-DB) were 0.53, 1.64 and 0.47 cm**2 yr**-1 in the northern, western and central abyssal Arabian Sea, respectively. Mobile epibenthic megafauna at the western, northern, central and southern study sites were dominated by ophiuroids, holothurians, ophiuroids and natant decapods (the respective densities were 100, 82, 29 and 6 individuals 1000 m**-2). The northern study site was characterized by a high abundance of spoke traces and fecal casts. The central site showed spoke traces and many tracks. The southern site displayed the highest abundance of spoke traces, whereas at the western site hardly any lebensspuren were observed. There is evidence for at least two functional endmember communities in the Arabian Sea. In the northwestern Arabian Sea (WAST) vertical particle displacement seems to be dominated by macrofauna and primarily eddy-diffusive. In the southern Arabian Sea (SAST) non-local and 'incidental' mixing due to spoke-trace producers might become more important and superimpose reduced eddy-diffusive mixing. With respect to biological data CAST is an intermediate location. Given the biological data, average 210Pb-DB is higher and decimeter-scale variability of 210Pb-DB smaller at CAST than expected. These findings indicate that in a mixture of both endmember communities the organisms may interact in way that increases values of biodiffusivity, as reflected by 210Pb-DB, and reduces decimeter-scale 210Pb-DB heterogeneity in comparison to the simple sum of the isolated effects of the endmembers. For time scales <100 years there was no evidence for a relationship between food supply (POC flux) and bioturbation intensity, as reflected by 210Pb-DB and 234Th-DB. Bioturbation intensity should be controlled primarily by the composition of the benthic fauna, its specific adaptation to the environmental setting, and the abundance of each species of the benthic community. Food supply can have only an indirect influence on bioturbation intensity. In certain parts of the ocean the a priori overall positive relationship between POC flux and biodiffusivity might include restricted intervals displaying no or even negative relations.

Anundance of phytoplankton at time series station DYNAPROC

A general study of biogeochemical processes (DYNAPROC cruise) was conducted in May 1995 at a time-series station in the open northwestern Mediterranean Sea where horizontal advection was weak. Short-term variations of the vertical distributions of pico- and nanophytoplankton were investigated over four 36-h cycles, along with parallel determinations of metabolic CO2 production rates and amino acid-containing colloid (AACC) concentrations at the chlorophyll maximum depth. The vertical (0-1000-m depth) distributions of (i) AACC, (ii) suspended particles and (iii) metabolic CO2 production rate were documented during the initial and final stages of these 36-h cycles. This study was concerned with diel vertical migration (DVM) of zooplankton, which provided periodic perturbations. Accordingly, the time scale of the experimental work varied from a few hours to a few days. Although all distributions exhibited a periodic behaviour, AACC distributions were generally not linked to diel vertical migrations. In the subsurface layer, Synechococcus made the most abundant population and large variations in concentration were observed both at day and at night. The corresponding integrated (over the upper 90 m) losses of Synechococcus during one night pointed to a potential source of exported organic matter amounting to 534 mg C/m**2. This study stresses the potential importance of organic matter export from the euphotic zone through the daily grazing activity of vertically migrating organisms, which would not be accounted for by measurements at longer time scales. The metabolic CO2 production exhibited a peak of activity below 500 m that was shifted downward, apparently in a recurrent way and independently of the vertical distributions of AACC or of suspended particulate material. To account for this phenomenon, a 'sustained wave train» hypothesis is proposed that combines the effect of the diel superficial faecal pellet production by swarming migrators and the repackaging activity of the nonmigrating midwater populations. Our results confirm the recent finding that the particulate compartment is not the major source of the observed instantaneous remineralisation rate and shed a new light on the fate of organic matter in the aphotic zone.

Biogenic opal in Late Pleistocene sediments of ODP Leg 177 holes

Sediments in the southeast Atlantic sector of the Southern Ocean were cored during Ocean Drilling Program (ODP) Leg 177 to study the paleoceanographic history of the Antarctic region on short (millennial) to long (Cenozoic) timescales. Seven sites were drilled along a north-south transect across the Antarctic Circumpolar Current (ACC) from 41° to 53°S. The general goals of Leg 177 were twofold: (1) to document the biostratigraphic, biogeographic, and paleoceanographic history of the Paleogene and early Neogene, a period marked by the establishment of the Antarctic cryosphere and the ACC, and (2) to target expanded sections of late Neogene sediments, which can be used to resolve the timing of Southern Hemisphere climatic events on orbital and suborbital time scales (Gersonde, Hodell, Blum, et al., 1999, doi:10.2973/odp.proc.ir.177.1999). Closely spaced measurements of sedimentary physical properties were obtained from all cores recovered during Leg 177 using the ODP whole-round multisensor track. In addition, high-resolution diffuse color reflectance and resistivity measurements were collected on the Oregon State University Split Core Analysis Track. These whole-core and split-core measurements provide high-resolution proxy data sets for the estimation of biogenic and terrigenous mineralogy and mass flux. To assist investigators in calibrating these proxy data sets from sites located within the circum-Antarctic opal belt, samples from Sites 1093 (50°S) and 1094 (53°S) were analyzed for biogenic opal content.

Benthic foraminiferal stable isotope stratigraphy of ODP Site 138-846 in the tropical Pacific Ocean

A stable-isotope stratigraphy at Site 846 (tropical Pacific, 3°06'S, 90°49'W, 3307 m water depth), based on the benthic foraminifers Cibicides wuellerstorfi and Uvigerina peregrina, yields a high-resolution record of deep-sea delta18O and delta13C over the past 1.8 Ma, with an average sampling interval of 3 k.y. Variance in the delta18O and delta13C records is concentrated in the well-known orbital periods of 100, 41, and 23 k.y. In the 100-k.y. band, both isotopic signals grow from relatively low amplitudes prior to 1.2 Ma, to high amplitudes in the late Quaternary since 0.7 Ma. The amplitude of delta18O and especially of delta13C decreases in the 41-k.y. band as it grows in the 100-k.y. band, consistent with a transfer of energy into an orbitally-paced internal oscillation. A weak 30-k.y. rhythm, present in both delta18O and delta13C, may reflect nonlinear interaction between the 41-k.y. and 100-k.y. bands in the evolving climate system. In the 23-k.y. and 19-k.y. bands associated with orbital precession, delta18O and delta13C are not coherent with each other on long time scales, and do not evolve like the 100-k.y. and 41-k.y. bands. This suggests that the source of the growing 100-k.y. oscillation is not a nonlinear response to precession, in contrast to predictions of some climate models. Sedimentation rates at this site also vary with a strong 100-k.y. cycle. Unlike the isotope records, the amplitude of 100-k.y. variations in sedimentation rate is relatively constant over the past 1.8 Ma, ranging from about 15 to 70 m/m.y. Prior to 0.9 Ma, sedimentation rates co-vary with orbital eccentricity, rather than with global climate as reflected by delta18O or delta13C. A source of this 100-k.y. cycle of sedimentation rate in the absence of similar ice volume fluctuations may be precessional heating of equatorial land masses, which in an energy balance climate model drives variations of monsoonal climates with a 100-k.y. rhythm. For the interval younger than 0.9 Ma, high sedimentation rates in the 100-k.y. band are consistently associated with glacial stages. This change of pattern suggests that when the amplitude of glacial cycles become large enough, their global effects overpower a local monsoon-driven variation in sedimentation rate at Site 846.

Paleomagnetic of ODP Hole 197-1203A

On the basis of studies of Holocene samples,submarine basaltic glass (SBG) is thought to be an ideal paleointensity recorder because it contains unaltered single domain magnetic inclusions that yield Thellier paleointensity data of exceptional quality. To be useful as a recorder of the long-term geomagnetic field, older SBG must retain these optimal properties. Here, we examine this issue through rock magnetic and transmission electron microscope (TEM) analyses of Cretaceous SBG recovered at Ocean Drilling Program Site 1203 (northwestern Pacific Ocean). These SBG samples have very low natural remanent magnetization intensities (NRM <50 nAm**2/g) and TEM analyses indicate a correspondingly low concentration of crystalline inclusions. Thellier experiments on samples with the strongest NRM intensity (>5*10**-11 Am**2) show a rapid acquisition of thermoremanent magnetization (TRM) with respect to NRM demagnetization. Taken at face value,this behavior implies magnetization in a very weak (617 WT) ambient field. But monitoring of magnetic hysteresis properties during the Thellier experiments (on subsamples of the SBG samples used for paleointensity determinations) indicates systematic variations in values over the same temperature range where the rapid TRM acquisition is observed. A similar change in properties during heating is observed on monitor SBG specimens using low-temperature data: with progressive heatings the Verwey transition becomes more distinct. We suggest that these experimental data record the partial melting and neocrystallization of magnetic grains in SBG during the thermal treatments required by the Thellier method,resulting in paleointensity values biased to low values. We further propose that this process is pronounced in Cretaceous and Jurassic SBG (relative to Holocene SBG) because devitrification on geologic time scales (i.e., tens of millions of years) lowers the transition temperature at which the neocrystallization can commence. Magnetic hysteresis monitoring may provide a straightforward means of detecting the formation of new magnetic inclusions in SBG during Thellier experiments.