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

Taylor, Sarah Louise (2008) Trace element distributions in ridge flank sediments from the east Pacific Rise, and their use as proxies of past ocean conditions. University of Southampton, School of Ocean and Earth Science, Doctoral Thesis, 152pp.

Record type: Thesis (Doctoral)

Abstract

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