Trace element distributions in ridge flank sediments from the east Pacific Rise, and their use as proxies of past ocean conditions

The eastern equatorial and tropical Pacific regions are areas of significant carbon fluxes from the<br/>atmosphere to the ocean interior. Changes in the function of marine biogeochemical cycles in this<br/>region potentially exert an important control on global climate. Understanding controls on and<br/>changes to ocean chemistry and circulation in this region is therefore of great importance.<br/>Redox sensitive metal distributions in hydrothermal sediments have yet to be exploited effectively<br/>as proxies of past ocean conditions. This work presents a 740 ka sediment record from an archived<br/>core collected at 14o47’S overlying 1.1 Ma crust on the western flank of the EPR. The<br/>metalliferous sedimentation is overprinted by diagenetic mobilisation arising from variations in the<br/>sediment redox status of the sediments. Amorphous ferrihydrite phases delivered to the sediment<br/>have undergone significant alteration to more stable crystalline forms. Under glacial conditions, the<br/>transformation of ferrihydrite appears to be impeded, which is inferred to be a function of a distinct<br/>change in the redox status of the sediments under glacial conditions.<br/>Oxyanions coprecipitated with Fe and Mn (hydr)oxides from the hydrothermal plume (P, V, U) are<br/>partitioned during Fe-oxide alteration. V is preferentially incorporated into goethite and residual<br/>phases, locking the plume derived V within the sediment. Sediment P/Fe ratios are lower than<br/>overlying plume values, and vary systematically with variations in ferrihydrite transformation to<br/>goethite on glacial-interglacial timescales. This transformation is inferred to lead to P loss from the<br/>particulate/sediment phase. Uranium is highly enriched in sulphide rich EPR plume particles and<br/>the sediments at 14oS. U/Fe ratios indicate there has been enhanced release of U under interglacial<br/>conditions, and preservation of plume U/Fe ratios under glacial conditions. Mo/Mn ratios are used<br/>to confirm the changes in redox status on glacial-interglacial timescales at this site. There is a<br/>general trend over Marine Isotope Stages 1-14 of a deepening of the sediment redox front through<br/>interglacial stages with a shallowing at the onset of glaciation. Enhanced sub-oxic conditions<br/>associated with glacial conditions (in particular MIS 12) are attributed to enhanced productivity<br/>(and carbon export to the seafloor) and decreased bottom water O2 (and therefore reduced<br/>ventilation of the deep water). This is consistent with paleoproductivity data from other parts of the<br/>Eastern Pacific and adds new information of past conditions in a region which has not been studied.

Name dc:type.qualificationname

Ph.D.

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Southampton

Year dc:date.issued

2008

Author dc:creator

• Taylor, Sarah Louise