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.

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.

(Table 1) Activity of alkali phosphatase in suspended matter at 20°C and environmental temperatures, and recycling time of organic phosphorus in the Barents and Norwegian seas

Alkali phosphatase activity and hydrochemical structure of waters in the Barents and Norwegian seas were investigated. In a sea with the seasonal bioproduction cycle alkali phosphatase activity is also seasonal, rising with trophic level of waters. At the end of hydrological and biological winter activity is practically zero. Alkali phosphatase activity is especially important in summer, when plankton has consumed winter supply of phosphate in the euphotic layer and nutrient limitation of primary production begins. In summer production and destruction cycle, apparent time for recycling of phosphorus by phosphatase in suspended matter in the euphotic layer of the Barents Sea and Norwegian Sea averages from 7 to 30 hours.

Iron and phosphorus speciations in DSDP Leg 92 sediments

We present a detailed study of the co-diagenesis of Fe and P in hydrothermal plume fallout sediments from ~19°S on the southern East Pacific Rise. Three distal sediment cores from 340-1130 km from the ridge crest, collected during DSDP Leg 92, were analysed for solid phase Fe and P associations using sequential chemical extraction techniques. The sediments at all sites are enriched in hydrothermal Fe (oxyhydr)oxides, but during diagenesis a large proportion of the primary ferrihydrite precipitates are transformed to the more stable mineral form of goethite and to a lesser extent to clay minerals, resulting in the release to solution of scavenged P. However, a significant proportion of this P is retained within the sediment, by incorporation into secondary goethite, by precipitation as authigenic apatite, and by readsorption to Fe (oxyhydr)oxides. Molar P/Fe ratios for these sediments are significantly lower than those measured in plume particles from more northern localities along the southern East Pacific Rise, and show a distinct downcore decrease to a depth of ~12 m. Molar P/Fe ratios are then relatively constant to a depth of ~35 m. The Fe and P speciation data indicate that diagenetic modification of the sediments is largely complete by a depth of 2.5 m, and thus depth trends in molar P/Fe ratios can not solely be explained by losses of P from the sediment by diffusion to the overlying water column during early diagenesis. Instead, these sediments are likely recording changes in dissolved P concentrations off the SEPR, possibly as a result of redistribution of nutrients in response to changes in oceanic circulation over the last 10 million years. Furthermore, the relatively low molar P/Fe ratios observed throughout these sediments are not necessarily solely due to losses of scavenged P by diffusion to the overlying water column during diagenesis, but may also reflect post-depositional oxidation of pyrite originating from the volatile-rich vents of the southern East Pacific Rise. This study suggests that the molar P/Fe ratio of oxic Fe-rich sediments may serve as a proxy of relative changes in paleoseawater phosphate concentrations, particularly if Fe sulfide minerals are not an important component during transport and deposition.