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 sediment sampled along two cross-Equator latitudinal transects at 135W and 140W in the central equatorial Pacific Ocean. Although traditionally both Al and Ti are considered to reside exclusively within terrigenous phases in marine sediment, these sediments present extremely high Al/Ti ratios that are several times that of average shale and other potential crustal sources. A sharp maximum in Al/Ti is observed slightly south of the Equator, where sedimentary bulk accumulation rate (BAR) is also highest (reflecting elevated productivity in the overlying water caused by surface water divergence). Bulk Al/Ti decreases sharply away (+/- 2 degrees latitude) to near crustal values at similar to 4 degrees north and south. The latitudinal profiles of Al/Ti are entirely unrelated to the concentration of the biogenic components as well as to the absolute accumulation of Al and Ti. These results indicate the presence of a significant scavenged component of Al sourced directly from seawater during particle settling. The data from the two transects analytically and oceanographically confirms our earlier work that was based 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 biogenic sediment, suspended particulate matter, and sediment trap material. Quantitative use of Al as an index of terrigenous material may, therefore, lead to an overstimation, by a factor of two, of the true terrigenous load in marine sediment, sedimentary rock, and settling particles. Because bulk Al/Ti appears to respond to sedimentary BAR, which in biogenic regimes is linked to surface water productivity, downcore records of Al/Ti in biogenic sediment may track productivity changes through 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.