Kelly A Kryc

Al-to-oxide and Ti-to-organic linkages in biogenic sediment: relationships to paleo-export production and bulk Al/Ti

Abstract To increase our understanding of the mechanisms that control the distribution of Al and Ti within marine sediment, we performed sequential extractions targeting the chemical signatures of the loosely bound, exchangeable, carbonate, oxide, organic, opal, and residual fraction of sediment from a carbonate-dominated regime (equatorial Pacific) and from a mixed opal–terrigenous regime (West Antarctic Peninsula). We observe a systematic partitioning of Al and Ti between sediment phases that is related to bulk Al/Ti. We show that, where we can quantify an Alexcess component, the dissolved Al is preferentially affiliated with the oxide fraction, resulting in Al/Ti molar ratios of 500–3000. This is interpreted as the result of surface complexation in the water column of dissolved Al onto oxyhydroxides. We also observe a previously undetected Tiexcess with as much as 80% of the total Ti in the organic fraction, which is most likely a function of metal-organic colloidal removal from the water column. In samples where the excess metals are obscured by the detrital load, the Al and Ti are almost exclusively found in the residual phase. This argues for the paired removal of Al (preferentially by the oxide component) and Ti (preferentially by the organic component) from the water column by settling particulate matter. This research builds upon earlier work that shows changes in the bulk ratio of Al to Ti in carbonate sediment from the central-equatorial Pacific that coincide with changes in the sedimentary bulk accumulation rate (BAR). The ratios that are observed are as much as three times higher than typical shale values, and were interpreted as the result of scavenging of dissolved Al onto particles settling in the water column. Because this non-terrigenous Alexcess accounts for up to 50% of the total sedimentary Al inventory and correlates best with BAR, the bulk Al/Ti may be a sensitive tracer of particle flux and, therefore, export production. Because we show that the excess metals are the result of scavenging processes, the bulk Al/Ti may be considered a sensitive proxy for this region.

Elemental fractionation of Si, Al, Ti, Fe, Ca, Mn, P, and Ba in five marine sedimentary reference materials: results from sequential extractions

Abstract We report on the elemental results from sequential extractions of BCSS-1 (marine sediment), MESS-1 (estuarine sediment), MAG-1 (marine mud), SCo-1 (Cody shale), and NIST-1c (argillaceous limestone) to encourage future comparisons of sequential extraction results within the marine geochemical and paleoceanographic communities. We measured Si, Al, Ti, Fe, Mn, Ca, P, and Ba in sequential de-ionized water (loosely-bound), MgCl2 (exchangeable), acetic acid (carbonate), hydroxylammonium hydrochloride (oxide), H2O2 (organic), Na2CO3 (opal), and residual (lithogenic) leaches. The protocol and selected elements were tailored to be most relevant to paleoceanographic geochemical studies instead of to environmental studies. Our results show that the sequential extraction procedure faithfully yields elemental distributions that are consistent with individual SRM lithologies. Our results also show that the procedure is typically reproducible within approximately 15%. However, in almost all cases, the procedure suffers from a systematic under-recovery of material when compared with the certified, bulk chemical analysis, and the under-recovery appears to be related to the lithology of the SRM. Similar under-recovery appears to be typical of sequential extraction procedures as reported by other previous studies. While this is problematic in assessing closure, it does not diminish the potential of inter-lab comparisons and first-order accuracy comparisons. We found that the elemental totals for the sequential extractions of MAG-1 compared best with the certified, bulk totals, and we recommend using this SRM to facilitate future accuracy assessments and inter-lab comparisons.

Pliocene export production and terrigenous provenance of the Southern Cape Basin, southwest African margin

Abstract We have analyzed 33 Pliocene bulk sediment samples from Ocean Drilling Program Site 1085 in the Cape Basin, located offshore of western Africa in the Angola–Benguela Current system, for 17 major and trace elements, and interpreted their associations and temporal variations in the context of an allied data set of CaCO3, opal, and Corg. We base our interpretations on elemental ratios, accumulation rates, inter-element correlations, and several multi-element statistical techniques. On the basis of qualitative assessment of downhole changes in the distributions of P and Ba, utilized as proxies of export production, we conclude that highs in bulk and biogenic accumulation that occur at 3.2 Ma, 3.0 Ma, 2.4 Ma, and 2.25 Ma were caused by increases in export production as well as terrigenous flux, and record a greater sequestering of organic matter during these time periods. Studies of refractory elements and other indicator proxies (SiO2, Al2O3, TiO2, Fe2O3, MgO, V, Cr, Sr, and Zr) strongly suggest that the terrigenous component of the bulk sediment is composed of two compositional end-members, one being ‘basaltic’ in composition and the other similar to an ‘average shale’. The basaltic end-member comprises approximately 10–15% of the total bulk sediment and its presence is consistent with the local geology of source material in the drainage basin of the nearby Orange River. The increase in bulk accumulation at 2.4 Ma appears to reflect a greater relative increase in basaltic input than the relative increase in shale-type input. Although studies such as this cannot precisely identify the transport mechanisms of the different terrigenous components, these results are most consistent with variations in sea level (and associated changes in shelf geometry and fluvial input) being responsible for the changing depositional conditions along the Angolan Margin during this time period.