SIMPLE-MODELS FOR THE GEOCHEMICAL RESPONSE OF THE OCEAN TO CLIMATIC AND TECTONIC FORCING

Simple models for the geochemical response of the ocean to periodic forcing are used as a framework to discuss the shortest periods of change resolvable by present analytical precision of elemental or isotopic ratio measurements of the chemical evolution of seawater. For chemical properties whose budget is dominated by fluxes into and out of the ocean, we show that the geochemical response to changing fluxes is strongly attenuated at periods shorter than the residence time, and that the response at these periods is also phase shifted so as to lag the forcing by one quarter cycle. The amplitude response as a function of period is used to estimate the periods of forcing resolvable by present analytical precision in measuring the Sr-87/Sr-86 and U-234/U-238 evolution of seawater, and we find that the shortest periods resolvable are of the order of 1 myr. If variations at periods shorter than this are found, we would argue that these are very likely of local, as opposed to global, origin. We illustrate the importance of phase shifts as a function of forcing period by discussing some recently published data that show a high degree of correlation (over the last 300 kyr) between changes in Sr-87/Sr-86 and deltaO-18 measured in the same sediment core. At first sight this correlation might be taken as good evidence that both Sr-87/Sr-86 and deltaO-18 are responding to some common climatic forcing, but given the long residence time of Sr in the ocean, Sr and O should in fact have different phase shifts relative to any proposed common forcing. Thus the apparent correlation between Sr-87/Sr-86 and deltaO-18 over the past several 100 Ma is not necessarily an indication that they are responding to a common cause. Amplitude response curves are also used to place limits on permissible global seawater U-234/U-238 variations, and these are then compared with published data to identify local (diagenetic) effects. Application to Os-187/Os-186 and U records in seawater detectors is proposed.