Royce W. Murray

Climatically sensitive eolian and hemipelagic deposition in the Cariaco Basin, Venezuela, over the past 578,000 years: Results from Al/Ti and K/Al

Al/Ti and K/Al ratios in bulk sediment are used to interpret wind-blown and hemipelagic sources of deposition to a 578 kyr record in the Cariaco Basin, Venezuela (Ocean Drilling Program site 1002). Graphical and cross-spectral analyses indicate that these ratios vary extremely closely with planktonic foraminiferal δ18O, with both ratios being significantly higher during interglacials and lower during glacials. K/Al indicates that during glacials the lower sea level that results in relative basin isolation increases the relative proportion of kaolinite derived from local rivers draining the relatively humid Venezuelan margin. Al/Ti decreases during glacials, suggesting greater proportions of eolian rutile sourced from the northern Sahara (due to increased wind strength and/or aridity). This interpretation is consistent with previous studies of the chemistry and mineralogy of Saharan-derived eolian matter in the Caribbean and with a mass balance determining the effect of changes in eolian rutile accumulation on the bulk sedimentary Al/Ti ratio.

Oxygenation history of bottom waters in the Cariaco Basin, Venezuela, over the past 578,000 years: Results from redox-sensitive metals (Mo, V, Mn, and Fe)

We present results from analyses of the redox-sensitive metals Mo, V, Mn, and Fe in sediment recovered from the Cariaco Basin (Ocean Drilling Program Leg 165, site 1002). Results are interpreted in the context of previous studies of δ15N, export production (percent total organic carbon), eolian input, and hemipelagic deposition in the basin. Variations in redox metal ratios over the past ∼578,000 years were compared to variations in δ18O at Milankovitch frequencies and show a strong relationship between glacial-interglacial cycles in sea level, governed by the shallow sills encircling Cariaco Basin, and bottom water oxygen content. During 100 kyr and 41 kyr cyclicity, enrichments of Mo and V occur during highly productive interglacials, indicating bottom water anoxia. During glacials, sediments are less depleted or enriched in Mn and Fe relative to the interglacials, reflecting oxic conditions. During 23 kyr and 19 kyr cyclicity, however, these redox metal patterns are not observed, indicating that the Cariaco Basin responds differently to the higher-frequency climate changes.

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.

Local and regional geochemical signatures of surface sediments from the Cariaco Basin and Orinoco Delta, Venezuela

We have analyzed the chemical compositions of 87 samples from the Cariaco Basin and 20 samples from the Orinoco Delta, Venezuela, in order to better characterize the fluvial sources of material to the Cariaco Basin. We observe distinctive regional patterns in composition; shelf sediments found near the Tuy River, Manzanares River, the Araya-Margarita region, and in the northern portion of the Unare platform record reproducible compositional differences. However, linear mixing models using only the composition from these rivers do not satisfy the variability observed in modern sediment trap samples collected from the basin. Average trap values are best approximated by mixing upper crust (30%–50%) with local river sources (50%–70% of either the Tuy, Unare, or Neveri Rivers) and eolian dust (