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In the central equatorial Pacific, settling particles collected with sediment traps have Al/Ti values greater than those of a typical crustal source. The Al/Ti values of the sediment trap material are not well correlated with either the flux of organic carbon or with total particle mass, suggesting that this ratio cannot be used as a simple index of paleoproductivity. However, the Al/Ti values of some trap samples are more than 3 times the ratio for typical crustal materials, indicating a noncrustal source of excess Al. Numerical analysis using linear programming indicates that the excess Al is associated with opal and that this biological component carries as much as 1200 ppm of Al. Because of the low particulate fluxes of lithogenic material in the equatorial Pacific and the relatively high opal content of equatorial sediment trap material, opal appears to be the predominant conveyor of Al to the equatorial sediments. Our sediment trap Al/Ti values are similar to ratios previously measured in equatorial sediments, suggesting that measurements of this element pair in ocean sediments may be an indicator of opal rain to the seafloor. The particle-reactive nature of Al and Ti means their ratios in sediments may be relatively insensitive to opal preservation or other diagenetic processes. However, the Al/Ti values of raining particles are highly sensitive to the relative proportion of opal and lithogenic material. Where opal/lithogenic ratios are less than 20, the Al/Ti values are indistinguishable from a crustal source ratio. In addition, the amount of excess Al associated with opal in settling particles may be a function of the concentration of Al in the water column and of the processes that fractionate dissolved Al and Ti. These issues must be evaluated before the Al/Ti values of ocean sediments can be used as a reliable proxy of opal flux to the seafloor. |
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