Amino acid in Deep Sea Drilling Project cores

Several amino acid diagenetic reactions, which take place in the deep-sea sedimentary environment, were investigated, using various Deep Sea Drilling Project (DSDP) cores. Initially it was found that essentially all the amino acids in sediments are bound in peptide linkages; but, with increasing age, the peptide bonds undergo slow hydrolysis that results in an increasingly larger fraction of amino acids in the free state. The hydrolysis half-life in calcareous sediments was estimated to be ~1-2 million years, while in non-carbonate sediment the hydrolysis rate may be considerably slower. The amino acid compositions and the extent of racemization of several amino acids were determined in various fractions isolated from the sediments. These analyses demonstrated that the mechanism, kinetics, and rate of amino acid diagenesis are highly dependent upon the physical state (i.e., free, bound, etc.) in which the amino acids exist in the sedimentary environment. In the free state, serine and threonine were found to decompose primarily by a dehydration reaction, while in the bound state (residue or HCl-insoluble fraction) a reversible aldol-cleavage reaction is the main decomposition pathway of these amino acids. The change in amino acid composition of the residue fraction with time was suggested to be due to the hydrolysis of peptide bonds, while in foraminiferal tests the compositional changes over geological time are the result of various decomposition reactions. Reversible first-order racemization kinetics are not observed for free amino acids in sediments. The explanation for these anomalous kinetics involves a complex reaction series which includes the hydrolysis of peptide bonds and the very rapid racemization of free amino acids. The racemization rates of free amino acids in sediments were found to be many orders of magnitude faster than those predicted from elevated temperature experiments using free amino acids in aqueous solution. The racemization rate enhancement of free amino acids in sediments may be due to the catalysis of the reaction by trace metals. Reversible first-order kinetics are followed for amino acids in the residue fraction isolated from sediments; the rate of racemization in this fraction is slower than that predicted for protein-bound amino acids. Various applications of amino acid diagenetic reactions are discussed. Racemization and the decomposition reaction of serine and threonine can both be used, with certain limitations, to make rough age estimates of deep-sea sediments back to several million years. The extent of racemization in foraminiferal tests which have been dated by some other independent technique can be used to estimate geothermal gradients, and thus heat flows, and to evaluate the bottom water temperature history in certain oceanic areas.

Deep water formation in the North Pacific and deglacial CO2 rise

Deep water formation in the North Atlantic and Southern Ocean is widely thought to influence deglacial CO2 rise and climate change; here we suggest that deep water formation in the North Pacific may also play an important role. We present paired radiocarbon and boron isotope data from foraminifera from sediment core MD02-2489 at 3640 m in the North East Pacific. These show a pronounced excursion during Heinrich Stadial 1, with benthic-planktic radiocarbon offsets dropping to ~350 years, accompanied by a decrease in benthic d11B. We suggest this is driven by the onset of deep convection in the North Pacific, which mixes young shallow waters to depth, old deep waters to the surface, and low-pH water from intermediate depths into the deep ocean. This deep water formation event was likely driven by an increase in surface salinity, due to subdued atmospheric/monsoonal freshwater flux during Heinrich Stadial 1. The ability of North Pacific Deep Water (NPDW) formation to explain the excursions seen in our data is demonstrated in a series of experiments with an intermediate complexity Earth system model. These experiments also show that breakdown of stratification in the North Pacific leads to a rapid ~30 ppm increase in atmospheric CO2, along with decreases in atmospheric d13C and D14C, consistent with observations of the early deglaciation. Our inference of deep water formation is based mainly on results from a single sediment core, and our boron isotope data are unavoidably sparse in the key HS1 interval, so this hypothesis merits further testing. However we note that there is independent support for breakdown of stratification in shallower waters during this period, including a minimum in d15N, younging in intermediate water 14C, and regional warming. We also re-evaluate deglacial changes in North Pacific productivity and carbonate preservation in light of our new data, and suggest that the regional pulse of export production observed during the Bølling-Allerød is promoted by relatively stratified conditions, with increased light availability and a shallow, potent nutricline. Overall, our work highlights the potential of NPDW formation to play a significant and hitherto unrealized role in deglacial climate change and CO2 rise.

Sedimentology of ODP Leg 126 deep-water volcaniclastics

Three sites were drilled in the Izu-Bonin forearc basin during Ocean Drilling Program (ODP) Leg 126. High-quality formation microscanner (FMS) data from two of the sites provide images of part of a thick, volcaniclastic, middle to upper Oligocene, basin-plain turbidite succession. The FMS images were used to construct bed-by-bed sedimentary sections for the depth intervals 2232-2441 m below rig floor (mbrf) in Hole 792E, and 4023-4330 mbrf in Hole 793B. Beds vary in thickness from those that are near or below the resolution of the FMS tool (2.5 cm) to those that are 10-15 m thick. The bed thicknesses are distributed according to a power law with an exponent of about 1.0. There are no obvious upward thickening or thinning sequences in the bed-by-bed sections. Spaced packets of thick and very thick beds may be a response to (1) low stands of global sea level, particularly at 30 Ma, (2) periods of increased tectonic uplift, or (3) periods of more intense volcanism. Graded sandstones, most pebbly sandstones, and graded to graded-stratified conglomerates were deposited by turbidity currents. The very thick, mainly structureless beds of sandstone, pebbly sandstone, and pebble conglomerate are interpreted as sandy debris-flow deposits. Many of the sediment gravity flows may have been triggered by earthquakes. Long recurrence intervals of 0.3-1 m.y. for the very thickest beds are consistent with triggering by large-magnitude earthquakes (M = 9) with epicenters approximately 10-50 km away from large, unstable accumulations of volcaniclastic sand and ash on the flanks of arc volcanoes. Paleocurrents were obtained from the grain fabric of six thicker sandstone beds, and ripple migration directions in about 40 thinner beds; orientations were constrained by the FMS images. The data from ripples are very scattered and cannot be used to specify source positions. They do, however, indicate that the paleoenvironment was a basin plain where weaker currents were free to follow a broad range of flow paths. The data from sandstone fabric are more reliable and indicate that turbidity currents flowed toward 150? during the time period from 28.9 to 27.3 Ma. This direction is essentially along the axis of the forearc basin, from north to south, with a small component of flow away from the western margin of the basin.

Geochemistry of terrigenous material in deep-sea sediments of the equatorial Pacific

Biological productivity in the modern equatorial Pacific Ocean, a region with high nutrients and low chlorophyll, is currently limited by the micronutrient Fe. In order to test whether Fe was limiting in the past and to identify potential pathways of Fe delivery that could drive Fe fertilization (i.e., dust delivery from eolian inputs vs. Fe supplied by the Equatorial Undercurrent), we chemically isolated the terrigenous material from sediment along a cross-equatorial transect in the central equatorial Pacific at 140°W and at Ocean Drilling Program Site 850 in the eastern equatorial Pacific. We quantified the contribution from each potential Fe-bearing terrigenous source using a suite of chemical- and isotopic discrimination strategies as well as multivariate statistical techniques. We find that the distribution of the terrigenous sources (i.e., Asian loess, South American ash, Papua New Guinea, and ocean island basalt) varies through time, latitude, and climate. Regardless of which method is used to determine accumulation rate, there also is no relationship between flux of any particular Fe source and climate. Moreover, there is no connection between a particular Fe source or pathway (eolian vs. Undercurrent) to total productivity during the Last Glacial Maximum, Pleistocene glacial episodes, and the Miocene "Biogenic Bloom". This would suggest an alternative process, such as an interoceanic reorganization of nutrient inventories, may be responsible for past changes in total export in the open ocean, rather than simply Fe supply from dust and/or Equatorial Undercurrent processes. Additionally, perhaps a change in Fe source or flux is related to a change in a particular component of the total productivity (e.g., the production of organic matter, calcium carbonate, or biogenic opal).

Chemical composition of dredged rocks from the Hess Deep, Eastern Equatorial Pacific

A joint analysis of data on the anomalous magnetic field, seismicity, and structures of the Hess deep basalts have allowed to specify propagation of the spreading zone and to correct position of the neovolcanic zone. A precise petrogeochemical analysis of various types of basalts composing the uneven-aged oceanic crust of the basin showed that magmatics of the neovolcanic zone are related to the primitive type in contrast to rift boards of differential basalts. A model of the deep structure of the Galapagos rift in the area of the western Hess Deep has been suggested.

Radioisotopes and He accumulation in deep sea sediments of the Ontong Java Plateau

The accumulation of extraterrestrial 3He, a tracer for interplanetary dust particles (IDPs), in sediments from the Ontong Java Plateau (OJP; western equatorial Pacific Ocean) has been shown previously to exhibit a regular cyclicity during the late Pleistocene, with a period of ~100 ka. Those results have been interpreted to reflect periodic variability in the global accretion of IDPs that, in turn, has been linked to changes in the inclination of Earth's orbit with respect to the invariable plane of the solar system. Here we show that the accumulation in OJP sediments of authigenic 230Th, produced by radioactive decay of 234U in seawater, exhibits a 100-ka cyclicity similar in phase and amplitude to that evident in the 3He record. We interpret the similar patterns of 230Th and 3He accumulation to reflect a common origin within the ocean-climate system. Comparing spatial and temporal patterns of sediment accumulation against regional patterns of biological productivity and against the well-established pattern of CaCO3 dissolution in the deep Pacific Ocean leads to the further conclusion that a common 100-ka cycle in accumulation of biogenic, authigenic and extraterrestrial constituents in OJP sediments reflects the influence of climate-related changes in sediment focusing, rather than changes in the rate of production or supply of sedimentary constituents.

Composition of dikes and lavas from the Pito Deep Rift

The northwest trending walls of the Pito Deep Rift (PDR), a tectonic window in the southeast Pacific, expose in situ oceanic crust generated ?3 Ma at the superfast spreading southern East Pacific Rise (SEPR). Whole rock analyses were performed on over 200 samples of dikes and lavas recovered from two ~8 km**2 study areas. Most of the PDR samples are incompatible-element-depleted normal mid-ocean ridge basalts (NMORB; (La/Sm)N < 1.0) that show typical tholeiitic fractionation trends. Correlated variations in Pb isotope ratios, rare earth element patterns, and ratios of incompatible elements (e.g., (Ce/Yb)N) are best explained by mixing curves between two enriched and one depleted mantle sources. Pb isotope compositions of most PDR NMORB are offset from SEPR data toward higher values of 207Pb/204Pb, suggesting that an enriched component of the mantle was present in this region in the past ?3 Ma but is not evident today. Overall, the PDR crust is highly variable in composition over long and short spatial scales, demonstrating that chemically distinct lavas and dikes can be emplaced within the same segment over short timescales. However, the limited spatial distribution of high 206Pb/204Pb samples and the occurrence of relatively homogeneous MgO compositions (ranging <2.5 wt %) within a few of the individual dive transects (over distances of ~1 km) suggests that the mantle source composition evolved and magmatic temperatures persisted over timescales of tens of thousands of years. The high degree of chemical variability between pairs of adjacent dikes is interpreted as evidence for along-axis transport of magma from chemically distinct portions of the melt lens. Our findings suggest that lateral dike propagation occurs to a significant degree at superfast spreading centers.

Oxygen, hydrogen, and carbon isotopic compositions in deep sea cherts, porcellanites, and coexisting carbonates from DSDP Legs 2, 3, 6, 7, 11, 14, 16, 17, and 20

Seventy four samples of DSDP recovered cherts of Jurassic to Miocene age from varying locations, and 27 samples of on-land exposed cherts were analyzed for the isotopic composition of their oxygen and hydrogen. These studies were accompanied by mineralogical analyses and some isotopic analyses of the coexisting carbonates. d18O of chert ranges between 27 and 39%. relative to SMOW, d18O of porcellanite - between 30 and 42%. The consistent enrichment of opal-CT in porcellanites in 18O with respect to coexisting microcrystalline quartz in chert is probably a reflection of a different temperature (depth) of diagenesis of the two phases. d18O of deep sea cherts generally decrease with increasing age, indicating an overall cpoling of the ocean bottom during the last 150 m.y. A comparison of this trend with that recorded by benthonic foraminifera (Douglas and Savin, 1975; http://www.deepseadrilling.org/32/volume/dsdp32_15.pdf) indicates the possibility of d18O in deep sea cherts not being frozen in until several tens of millions of years after deposition. Cherts of any Age show a spread of d18O values, increasing diagenesis being reflected in a lowering of d18O. Drusy quartz has the lowest d18O values. On-land exposed cherts are consistently depleted in 18O in comparison to their deep sea time equivalent cherts. Water extracted from deep sea cherts ranges between 0.5 and 1.4 wt %. dD of this water ranges between -78 and -95%. and is not a function of d18O of the cherts (or the temperature of their formation).

Interstitial water chemistry, Deep Sea Drilling Project, Leg 96

On Leg 96 of the Deep Sea Drilling Project (DSDP), holes were drilled in Orca and Pigmy basins on the northern Gulf of Mexico continental slope and on the Mississippi Fan. The holes on the fan encountered interbedded sand, silt, and mud deposited extremely rapidly, most during late Wisconsin glacial time. Pore-water chemistry in these holes is variable, but does not follow lithologic changes in any simple way. Both Ca and SO4 are enriched in the pore water of many samples from the fan. Two sites drilled in the prominent central channel of the middle fan show rapid SO4 reduction with depth, whereas two nearby sites in overbank deposits show no sulfate reduction for 300 m. Calcium concentration decreases as SO4 is depleted and Li follows the same pattern. Strontium, which like Li, is enriched in samples enriched in Ca, does not decrease with SO4 and Ca. Potassium in the pore water decreases with depth at almost all sites. Sulfate reduction was active at the two basin sites and, as on the fan, this resulted in calcium carbonate precipitation and a lowering of pore water Ca, Mg, and Li. The Orca Basin site was drilled through a brine pool of 258? salinity. Pore-water salinity decreases smoothly with depth to 50 m and remains well above normal seawater values to the bottom of the hole at about 90 m. This suggests constant sedimentation under anoxic hypersaline conditions for at least the last 50,000 yr.