Carbonate content in lower Cretaceous sediments of ODP Hole 123-765C

Sediments recovered during Ocean Drilling Program (ODP) Leg 123 from the Argo Abyssal Plain (AAP) consist largely of turbidites derived from the adjacent Australian continental margin. The oldest abundant turbidites are Valanginian-Aptian in age and have a mixed (smarl) composition; they contain subequal amounts of calcareous and siliceous biogenic components, as well as clay and lesser quartz. Most are thin-bedded, fine sand- to mud-sized, and best described by Stow and Piper's model (1984) for fine-grained biogenic turbidites. Thicker (to 3 m), coarser-grained (medium-to-coarse sand-sized) turbidites fit Bouma's model (1962) for sandy turbidites; these generally are base-cut-out (BCDE, BDE) sequences, with B-division parallel lamination as the dominant structure. Parallel laminae most commonly concentrate quartz and/or calcispheres vs. lithic clasts or clay, but distinctive millimeter- to centimeter-thick, radiolarian-rich laminae occur in both fine- and coarse-grained Valanginian-Hauterivian turbidites. AAP turbidites were derived from relatively deep parts of the continental margin (outer shelf, slope, or rise) that lay below the photic zone, but above the calcite compensation depth (CCD). Biogenic components are largely pelagic (calcispheres, foraminifers, radiolarians, nannofossils); lesser benthic foraminifers are characteristic of deep-water (abyssal to bathyal) environments. Abundant nonbiogenic components are mostly clay and clay clasts; smectite is the dominant clay species, and indicates a volcanogenic provenance, most likely the Triassic-Jurassic volcanic suite exposed along the northern Exmouth Plateau. Lower Cretaceous smarl turbidites were generated during eustatic lowstands and may have reached the abyssal plain via Swan Canyon, a submarine canyon thought to have formed during the Late Jurassic. In contrast to younger AAP turbidites, however, Lower Cretaceous turbidites are relatively fine-grained and do not contain notably older reworked fossils. Early in its history, the northwest Australian margin provided mainly contemporaneous slope sediment to the AAP; marginal basins adjacent to the continent trapped most terrigenous detritus, and pronounced canyon incisement did not occur until Late Cretaceous and, especially, Cenozoic time.

Carbonate preservation in marine sediments

Observations of carbonate preservation in marine sediments have long been used to infer changes in ocean circulation or biogenic production. When combined with measures of organic carbon rain and calcite accumulation rates, quantitative estimates of changes in preservation can reveal variation in biogenic fluxes, the org. C to calcite flux ratio and saturation state of bottom waters. Here we develop quantitative dissolution proxies for mid to higher latitudes based on foraminiferal test fragmentation. Examining surface sediments, we find that fragmentation in G. bulloides and G. truncatulinoides is linear with increasing seabed dissolution rate and can be used to quantify changes in carbonate preservation. G. truncatulinoides shows a constant relationship of fragmentation to dissolution. However, we observe that, although linear to dissolution rate, the fragmentation in G. bulloides depends on which morphotype is present. Other species, such as G. inflata, have complex responses to increasing dissolution and are less direct preservation indicators.

Calcium carbonate content and concentration of phosphorus components in sediments of ODP Leg 171B sites

Quantifying phosphorus (P) concentrations in marine sediments is necessary for constraining the oceanic record of phosphorus burial and helps to constrain P sedimentary geochemistry. To understand P geochemistry in the sediments, we must determine the geochemical forms of P as well as the transformations occurring between these P components with depth and age. Although several records now exist of P geochemistry in the western and eastern equatorial Pacific (Filippelli and Delaney, 1995, doi:10.2973/odp.proc.sr.138.144.1995; 1996, doi:10.1016/0016-7037(96)00042-7), the western equatorial Atlantic (Delaney and Anderson, 1997, doi:10.2973/odp.proc.sr.154.124.1997), the California Current (Delaney and Anderson, in press), and the Benguela Current (Anderson et al., 2001, doi:10.1029/2000GB001270), most of these are Neogene records. Relatively little data exist from sediments of the Paleogene or Cretaceous, time periods when carbon isotope records indicate major carbon shifts and when the nature of P geochemistry has not been well constrained. Samples from several sites at various water depths, oceanographic regions, and ages are needed to understand how P geochemistry and burial in sediments reflect ocean history. We determined P geochemistry and reactive P concentrations in Atlantic sediments of Eocene to Cretaceous age. These are the first records of P geochemistry with good age control from this period. Blake Nose sites are ideal for investigating P geochemistry, as the sediments are shallowly buried at a range of water depths and sedimentation rates. We determined P concentrations and geochemistry, along with calcium carbonate contents, in mid-Cretaceous to upper Eocene sediments drilled on Blake Nose (Ocean Drilling Program Leg 171B) in a depth transect of four sites (Sites 1052, 1051, 1050, and 1049; water depths: 1345, 1983, 2300, and 2656 m, respectively).

Phosphorus components and barite in untreated and in carbonate-free sediments of ODP Hole 199-1221C

We determined changes in equatorial Pacific phosphorus (µmol P/g) and barite (BaSO4; wt%) concentrations at high resolution (2 cm) across the Paleocene/Eocene (P/E) boundary in sediments from Ocean Drilling Program (ODP) Leg 199 Site 1221 (153.40 to 154.80 meters below seafloor [mbsf]). Oxide-associated, authigenic, and organic P sequentially extracted from bulk sediment were used to distinguish reactive P from detrital P. We separated barite from bulk sediment and compared its morphology with that of modern unaltered biogenic barite to check for diagenesis. On a CaCO3-free basis, reactive P concentrations are relatively constant and high (323 µmol P/g or ~1 wt%). Barite concentrations range from 0.05 to 5.6 wt%, calculated on a CaCO3-free basis, and show significant variability over this time interval. Shipboard measurements of P and Ba in bulk sediments are systematically lower (by ~25%) than shore-based concentrations and likely indicate problems with shipboard standard calibrations. The presence of Mn oxides and the size, crystal morphology, and sulfur isotopes of barite imply deposition in sulfate-rich pore fluids. Relatively constant reactive P, organic C, and biogenic silica concentrations calculated on a CaCO3-free basis indicate generally little variation in organic C, reactive P, and biogenic opal burial across the P/E boundary, whereas variable barite concentrations indicate significant changes in export productivity. Low barite Ba/reactive P ratios before and immediately after the Benthic Extinction Event (BEE) may indicate efficient nutrient burial, and, if nutrient burial and organic C burial are linked, high relative organic C burial that could temporarily drawdown CO2 at this site. This interpretation requires postdepositional oxidation of organic C because organic C to reactive P ratios are low throughout the section. After the BEE, higher barite Ba/reactive P ratios combined with higher barite Ba concentrations may imply that higher export productivity was coupled with unchanged reactive P burial, indicating efficient nutrient and possibly also organic C recycling in the water column. If the nutrient recycling is decoupled from organic C, the high export production could be indicative of drawdown of CO2. However, the observation that organic C burial is not high where barite burial is high may imply that either C sequestration was restricted to the deep ocean and thus occurred only on timescales of the deep ocean mixing or that postdepositional oxidation (burn down) of organic matter affected the sediments. The decoupling of barite and opal may result from low opal preservation or production that is not diatom based.

(Table T1) Calcium carbonate content of sediments from ODP Leg 172 sites

Visual-domain diffuse reflectance data collected aboard the JOIDES Resolution with the Minolta spectrometer CM-2002 during Ocean Drilling Program Leg 172 have been used to estimate successfully the carbonate content of sediments. Calibration equations were developed for each site and for each lithostratigraphic unit (or subunit at Site 1063) using multiple linear regression on raw as well as pretreated reflectance spectra (i.e., first-order derivation and squaring of raw reflectance spectra) for a total of 4141 direct carbonate measurements. The root-mean-square errors of 4% to 7% are within the range of previous estimates using diffuse reflectance data and are acceptable for the general extensive range of carbonate contents (i.e., 0-70 wt%) that characterize sedimentation at Leg 172 sites.

Foraminifer, coarse fraction and carbonate characteristics of sediments from ODP Leg 198 sites on Shatsky Rise

Holes 1209A and 1211A on Southern High, Shatsky Rise contain expanded, nearly continuous records of carbonate-rich sediment deposited in deep water of the equatorial Pacific Ocean during the Paleocene and Eocene. In this study, we document intervals of carbonate dissolution in these records by examining temporal changes in four parameters: carbonate content, coarse size fraction (>38 µm), benthic foraminiferal abundance, and planktonic foraminiferal fragmentation ratio. Carbonate content is not a sensitive indicator of carbonate dissolution in the studied sections, although rare intervals of low carbonate may reflect times of relatively high dissolution. The proportion of coarse size fraction does not accurately record carbonate dissolution either because the relative abundance of nannofossils largely determines the grain-size distribution. Benthic abundance and fragmentation covary (r**2 = 0.77) and are probably the best indicators for carbonate dissolution. For both holes, records of these parameters indicate two episodes of prominent dissolution. The first of these occurs in the upper Paleocene (~59-58 Ma) and the second in the middle to upper Eocene (~45-33.7 Ma). Other intervals of enhanced carbonate dissolution are located in the upper Paleocene (~56 Ma) and in the upper lower Eocene (~51 Ma). Enhanced preservation of planktonic foraminiferal assemblages marks the start of both the Paleocene and Eocene epochs.

Chemical and isotope composition of carbonate nodules and host bottom sediments from Core DM9-666, Gulf of California

Results of mineralogical and isotopic analyzes of sulfur and carbon in carbonate nodules and host bottom sediments and results of 14C measurement in carbonate nodules are reported. It is proved that the carbonate nodules formed 11-22 thousand years ago in anaerobic diagenesis of bottom sediments rich in organic matter. Isotopic light metabolic carbon dioxide was a source of carbonate for nodules. It formed during microbial degradation of organic matter of bottom sediments.

Calcium carbonate, biogenic opal, and organic carbon content and d15N ratios of sediments from the Bering Sea

Millennial-scale paleoceanographic changes in the Bering Sea during the last 71 kyrs were reconstructed using geochemical and isotope proxies (biogenic opal, CaCO3, and total organic carbon (TOC), nitrogen and carbon isotopes of sedimentary organic matters) and microfossil (radiolaria and foraminifera) data from two cores (PC23A and PC24A) which were collected from the northern continental slope area at intermediate water depths. Biogenic opal and TOC contents were generally high with high sedimentation rates during the last deglaciation. Laminated sediment depositions during the Early-Holocene (EH) and Bølling-Allerød (BA) were closely related with the increased primary productivity recorded by high biogenic opal and TOC contents and high d15N values. Enhanced surface-water productivity was attributed to increased nutrient supply from strengthened Bering Slope Current (BSC) and from increased amount of glacial melt-water, resulting in high C/N ratios and low d13C values, and high proportion of Rhizoplegma boreale during the last deglaciation. In contrast, low surface-water productivity during the last glacial period was due to depleted nutrient supply caused by strong stratification and to restricted phytoplankton bloom by extensive sea ice distribution under cold climates. Extensive formation of sea ice produces more oxygen-rich intermediate-water, leading to oxic bottom-water conditions due to active ventilation, which favored good preservation of oxic benthic foraminifera species. Remarkable CaCO3 peaks coeval with high biogenic opal and TOC contents in both cores during MIS 3 to MIS 4 are most likely correlated with Dansgaard-Oeschger (D-O) events. High d15N and d13Corg values during D-O interstadials support increased surface-water productivity resulting from nutrients supplied mainly by intensified BSC. During the EH, BA and D-O interstadials, dominant benthic foraminifera species indicate dysoxic bottom-water conditions as a result of increased surface-water productivity and weak ventilation of intermediate-water with mitigated sea ice development caused by strengthening of the Alaskan Stream. It is of note that the bottom-water conditions and formation of intermediate-water in the Bering Sea during the last glacial period are related to the variation of dissolved oxygen concentration of the bottom-water in the northeastern Pacific and to strong ventilation of intermediate-water in the northwestern Pacific. Thus, the millennial-scale paleoceanographic events in the Bering Sea during the D-O interstadials are closely associated with the intermediate-water ventilation, ultimately leading to weakening of North Pacific Intermediate Water.

Carbonate system of surface sediments from the South Pacific

B/Ca ratios in Cibicides mundulus and Cibicides wuellerstorfi have been shown to correlate with the degree of calcite saturation in seawater (D[CO32-]). In the South Pacific, a region of high importance in the global carbon cycle, these species are not continuously present in down-core records. Small numbers of epibenthic foraminifera in samples present an additional challenge, which can be overcome by using laser ablation-inductively coupled-mass spectrometry (LA-ICP-MS). We present a laser ablation based core-top calibration for Cibicides cf. wuellerstorfi, a C. wuellerstorfi morphotype that is abundant in the South Pacific and extend the existing global core top calibration for C. mundulus and C. wuellerstorfi to this region. B/Ca in C. cf. wuellerstorfi are linearly correlated with D[CO32-] and possibly display a higher sensitivity to calcite saturation changes than C. wuellerstorfi. Trace element profiles through C. wuellerstorfi and C. mundulus reveal an intra-shell B/Ca variation of ±36% around the mean shell value. Mg/Ca and B/Ca display opposite trends along the shell. Both phenomena likely result from ontogenetic effects. Intra-shell variability equals intra-sample variability, mean sample B/Ca values can thus be reliably calculated from averaged spot results of single specimen. In the global B/Ca-D[CO32-] range, we observe an inverse relationship between water mass age and D[CO32-].

Carbonate minerals, uranium and thorium concentration and U-Th ages of sediments from ODP Leg 166 sites

We have performed U-Th isotope analyses on pure aragonite samples from the upper sections of Leg 166 cores to assign each aragonite-rich sediment package to the correct sea-level highstand. The uppermost sediment package from each of the four sites investigated (Sites 1003, 1005, 1006, and 1007) yielded a Holocene U-Th age. Sediment packages from deeper in the cores have suffered diagenesis. This diagenesis consists of significant U loss (up to 40%) in the site nearest the platform (Site 1005), slight U gain in sites further from the platform, and continuous loss of pure 234U caused by alpha recoil at all sites. The difference in diagenesis between the sites can be explained by the different fluid-flow histories they have experienced. Site 1005 is sufficiently close to the platform to have probably experienced a change in flow direction whenever the banks have flooded or become exposed. Other sites have probably experienced continuous flow into the sediment. Although diagenesis prevents assignment of accurate ages, it is sufficiently systematic that it can be corrected for and each aragonite-rich package assigned to a unique highstand interval. Site 1005 has sediment packages from highstands associated with marine isotope Stages 1, 5, 7, 9, and 11. Site 1006 is similar, except that the Stage 7 highstand is missing, at least in Hole 1006A. Site 1003 has sediment only from Stage 1 and 11 highstands within the U-Th age range. And Site 1007 has sediment only from the stage 1 highstand. This information will allow the construction of better age models for these sites. No high-aragonite sediments are seen for Stage 3 or Substages 5a and 5c. Unless rather unusual erosion has occurred, this indicates that the banks did not flood during these periods. If true, this would require the sea level for Substages 5a and 5c to have remained at least ~10 m lower than today.