Phosphorus components, trace metals and Ba excess of ODP Site 202-1237 sediments

We determined the sedimentary concentrations of phosphorus (P), barium (Ba), manganese (Mn), titanium (Ti), aluminum (Al), and uranium (U) for sediment samples from the southeast Pacific Nazca Ridge, Ocean Drilling Program Site 1237. This unique record extends to 31 Ma over 360 meters composite depth (mcd), recording depositional history as the site progressed eastward over its paleohistory. We sampled with a temporal resolution of ~0.2 m.y. throughout the sequence, equivalent to an average spacing of 1.63 m/sample. Concentrations of sequentially extracted components of P (oxide-associated, authigenic, organic, and detrital) increase toward the modern. Al/Ti ratios indicate that the background detrital source material is consistent with upper continental crust. U enrichment factors (U EFs) generally exceed crustal values and indicate slightly reducing environments. However, authigenic U precipitation can also be influenced by the organic carbon rain rate and may not be solely an indicator of redox conditions. Dramatic changes in Mn EFs at ~162 mcd, from values between 12 and 93 to values <12 after this depth, and a sharp color contact boundary lead us to believe that a paleoredox boundary from an oxygenated to a more reducing depositional environment occurred near this depth. Estimates of biogenic barite concentrations from a total sediment digestion technique (Ba excess) are greater than those from a barite extraction (Ba barite) for selected samples across the entire depth range. Applying a range of Ba/Ti ratios from different source materials to correct for detrital inputs does not change the lack of agreement with Ba barite concentrations. Reactive P (P reactive) concentrations (the sum of oxide-associated, authigenic, and organic P concentrations) increase toward the modern with values typically <12 µmol P/g from the base of our record through ~100 mcd, with a gradual increase to concentrations >15 µmol P/g. Ba excess follows the same general trends as Preactive, with concentrations <14 µmol Ba/g in the lower portion of the record to values >15 µmol Ba/g. Accumulation rate records of these proxies will be needed to infer paleoproductivity. P reactive/Ba excess ratios, an indicator of the relative burial of the nutrient P to organic carbon export, exhibit higher values, similar to modern, from the base of our record through ~180 mcd. The remainder of the record exhibits values lower than modern, indicating that organic carbon export to the sediments was higher relative to nutrient burial.

Radon 222 excess measured on water bottle samples during HEINCKE cruise HE153

In the years 2000 and 2001 we measured methane concentrations exceeding up to two orders of magnitude the equilibrium with the atmosphere in the water column on the SW-Spitsbergen continental shelf. This methane anomaly extended from its centre on the shelf westwards over the upper slope and eastwards well into the inner basins of the two southernmost Spitsbergen fjords, the Hornsundfjord and the van Mijenfjord. Methane concentrations and stable carbon isotopic ratios varied between 2 and 240 nM, and between -53 per mill and -20 per mill VPDB, respectively. Methane in high concentrations was depleted in 13C whereas in low concentrations d13CCH4 values were highly variable. On the continental shelf we found that methane discharged from seeps on top of sandy and gravelly banks is isotopically heavier than methane escaping from troughs filled with silty and clayey sediments. These distinct isotopic signatures suggest that methane is gently released from several inter-granular seepages or micro-seepages widely spread over the shelf. A potential migration path for thermogenic or hydrate methane may be the Hornsund Fracture Zone, a south-north running reactivated fault system created by stretching of the continental crust. After discharge into the water column, local water currents fed by Atlantic water, coastal water, and freshwater outflows from the fjords further determine pathways and fate of the methane. We used d18Owater and 222Rn data to trace origin and advection of the local water masses and water mixing processes. Methane spreads predominantly along pycnoclines and by vertical mixing. During transport methane is influenced simultaneously by oxidation and dilution, as well as loss into the atmosphere. Together these processes cause the spatial variability of the anomaly and heterogeneity in d13CCH4 in this polar shelf environment.

(Table 3) Barium barite and excess comparison, accumulation rates and productivity from surface sediments

Since Dymond et al. (1992, doi:10.1029/92PA00181) proposed the paleoproductivity algorithm based on "Bio-Ba", which relies on a strong correlation between Ba and organic carbon fluxes in sediment traps, this proxy has been applied in many paleoproductivity studies. Barite, the main carrier of particulate barium in the water column and the phase associated with carbon export, has also been suggested as a reliable paleoproductivity proxy in some locations. We demonstrate that Ba(excess) (total barium minus the fraction associated with terrigenous material) frequently overestimates Ba(barite) (barium associated with the mineral barite), most likely due to the inclusion of barium from phases other than barite and terrigenous silicates (e.g., carbonate, organic matter, opal, Fe-Mn oxides, and hydroxides). A comparison between overlying oceanic carbon export and carbon export derived from Ba(excess) shows that the Dymond et al. (1992) algorithm frequently underestimates carbon export but is still a useful carbon export indicator if all caveats are considered before the algorithm is applied. Ba(barite) accumulation rates from a wide range of core top sediments from different oceanic settings are highly correlated to surface ocean 14C and Chlorophyll a measurements of primary production. This relationship varies by ocean basin, but with the application of the appropriate f ratio to 14C and Chlorophyll a primary production estimates, the plot of Ba(barite) accumulation and carbon export for the equatorial Pacific, Atlantic, and Southern Ocean converges to a global relationship that can be used to reconstruct paleo carbon export.