SCAVENGED EXCESS ALUMINUM AND ITS RELATIONSHIP TO BULK TITANIUM IN BIOGENIC SEDIMENT FROM THE CENTRAL EQUATORIAL PACIFIC-OCEAN

We present results from chemical analyses of Aland Ti in surface sedim ent sampled along two cross-Equator latitudinal transects at 135W and 140W in the central equatorial Pacific Ocean. Although traditionally b oth Al and Ti are considered to reside exclusively within terrigenous phases in marine sediment, these sediments present extremely high Al/T i ratios that are several times that of average shale and other potent ial crustal sources. A sharp maximum in Al/Ti is observed slightly sou th of the Equator, where sedimentary bulk accumulation rate (BAR) is a lso highest (reflecting elevated productivity in the overlying water c aused by surface water divergence). Bulk Al/Ti decreases sharply away (+/- 2 degrees latitude) to near crustal values at similar to 4 degree s north and south. The latitudinal profiles of Al/Ti are entirely unre lated to the concentration of the biogenic components as well as to th e absolute accumulation of Al and Ti. These results indicate the prese nce of a significant scavenged component of Al sourced directly from s eawater during particle settling. The data from the two transects anal ytically and oceanographically confirms our earlier work that was base d on the single 135W transect. Calculations of Al-excess indicate that the highest Al/Ti ratios correspond to similar to 50% of the total Al being unsupported by the small amount of terrigenous phases present. These results are consistent with previous and ongoing studies of biog enic sediment, suspended particulate matter, and sediment trap materia l. Quantitative use of Al as an index of terrigenous material may, the refore, lead to an overstimation, by a factor of two, of the true terr igenous load in marine sediment, sedimentary rock, and settling partic les. Because bulk Al/Ti appears to respond to sedimentary BAR, which i n biogenic regimes is linked to surface water productivity, downcore r ecords of Al/Ti in biogenic sediment may track productivity changes th rough time. Such Al/Ti proxy records may be applicable in sediment of all ages, unlike radionuclide tracers which are limited by radioactive decay to use over the past hundreds of kyr.