Study On The Metalliferous Sediments Near13°N East Pacific Rise

Three box sediment cores (E271, E272and E53) collected from the flanks of13°NEast Pacific Rise were comprehensively studied on here. The three cores are coloredin red-brown in their upper layers and in yellow-green in their lower layers. Abundantof foraminifera occurred in the three cores when they were sampled onboard in2003,and they have disappeared with time. The>63micron fractions of two of the cores(E272, E271) are predominantly composed of diatoms. Some of the diatoms are intact,some are partially dissolved and some present as diatom fragments. Proportions ofdiatom fragments increasing with increasing depth have been observed, suggesting thedissolution of these diatoms must have occurred after burial. Grain-size data suggestthat the cores are composed of clayey silt with an average clay/silt ratio of0.5andnearly no sand fraction presents. The three cores are enriched in Fe, Mn, P and largenumbers of trace transition metals, showing obvious metalliferous characteristics.Early diagenesis exerted influences on them in different degree, under whichredox-sensitive elements, such as Mn, Co, have migrated upward. Sourceendmembers contributing to the three cores are predominantly biogenic carbonates,hydrothermal materials and terrigenous detrital. Biogenic Si and Ba as well ashydrogenic materials account for small percentages as well. Besides abundant of Fe-,Mn-oxides/hydroxides, the three cores contain significant quartz, feldspar, magnetite,calcite, salt, smectite, illite and kaolinite plus chlorite. The smectite is possiblyFe-smectite formed following the reaction between biogenic silica andFe-oxides/hydroxides under early diagenesis and it is probably an efficient sorbent forREE in deep-sea environment. Shale normalized REE pattern of bulk samplespresents characteristics of significant Ce negative anomalies, weak positive Euanomalies and relative HREE enrichment, suggesting seawater and hydrothermal fluidare the major REE sources for the sediments and seawater plays the most importantrole. The sedimentation rate of the three cores ranges from1.13-1.52cm/kyr, higher than the background sedimentation rate of semi-pelagic sediments in that area due tointensive hydrothermal input. The lower layers of Core E271must have been erodedby bottom currents, while Core E53must have been affected by such bottom currentsthroughout the core, resulting in large difference between its measured sedimentationrate and its theoretical sedimentation rate. Both the measured sedimentation rate andthe theoretical sedimentation rate can be reasonably interpreted. The measuredsedimentation rate reflects the ultimate materials accumulated in the sediments whilethe theoretical one reflects the intensity of potential hydrothermal influence. Theporosity of Core E272was studied on as well. Its downcore profile of porosity fitswell with the steady state compaction model. Exchanging mechanism of dissolvedmaterials between bottom seawater and the sediment at the water-sediment interface isdominated by molecular diffusion. However, under that interface, the mechanismcontrolling the transportation of dissolved materials is the directed flow of theinterstitial water from the bottom up, and this is consistent with the profiles of labileelements versus depth of this sediment core. This conclusion is most instructive forfurther study on the chemical diagenesis of this sediment core.