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doi:10.2204/iodp.proc.320321.203.2012

Results and discussion

One of the important uses of XRF scanning is to be able to produce high-resolution data sets of chemical composition for long time spans (e.g., the 18 m.y. sediment record at Site U1338). Figure F6 shows NMS time series of CaCO₃ and SiO₂. These two components average ~90% of the total sediment composition. Multiple intervals of low carbonate at Site U1338 are marked by tan bars on Figure F6. Each of these intervals have high SiO₂, caused by either the dissolution loss of carbonate from the sediment column or by the very fast deposition of an SiO₂-rich sediment component, like the late Miocene–Pliocene diatom mat deposits found in the eastern equatorial Pacific (Kemp and Baldauf, 1993; Expedition 320/321 Scientists, 2010b). One of the primary tasks of postexpedition studies is to understand how primary productivity and dissolution during early diagenesis have shaped the composition of the sediment column. Based on an average sedimentation rate of 25–30 m/m.y. below 60 revised meters composite depth (rmcd) in Site U1338 (see the “Site U1338” chapter [Expedition 320/321 Scientists, 2010b]) large-scale low weight percent CaCO₃ transients are spaced a million or more years apart. Smaller scale CaCO₃ variability is also observed at all the typical orbital forcing periods—400 to 19 k.y.

One way to study the aluminosilicate signal is with an aluminosilicate element like Ti. Because of low levels of aluminosilicates in the biogenic-rich equatorial Pacific sediments, it is more appropriate to use Ti than Al, because sufficient Al can be bound to bio-SiO₂ to cause an appreciable Al signal, especially at intervals in the sediment column where bio-SiO₂ was initially deposited at the seafloor but then partly dissolved during early diagenesis (Dymond et al., 1997). Figure F7 is a plot of TiO₂ and SiO₂ NMS concentrations along the Site U1338 splice. The blue intervals mark intervals where SiO₂ is very high but TiO₂ is essentially zero. These sediment intervals are extremely rich in bio-SiO₂. In the upper 60 m of the sediment column (tan interval), we observe higher TiO₂ associated with moderate SiO₂, indicating relatively more clays. This interval marks the last 5 m.y. of the record as the site moved north of 1.3°N to its present position at 2.6°N. The increase in clay component may represent increased dissolution of bio-SiO₂ and CaCO₃ as sedimentation slowed down when Site U1338 moved away from the equatorial high-productivity zone or increased dust deposition as Site U1338 moves toward the Intertropical Convergence Zone. The interval of elevated Ti coincides with the upper section of core with lower sedimentation rates (12.7 m/m.y. versus 28.7 m/m.y. below; see Fig. F14 in the “Site U1338” chapter [Expedition 320/321 Scientists, 2010b]). Mass accumulation rates of TiO₂, based on preliminary linear sedimentation rates, do not change with the increase in TiO₂%. The lack in change of flux makes the biogenic sediment dissolution hypothesis the most probable cause for TiO₂ enrichment in the upper section, not higher dust deposition. Supporting this interpretation, the modern position of Site U1338 is well south of the dust maximum associated with the Intertropical Convergence Zone, 5°–6° N since 5 Ma (Hovan, 1995).

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