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 fo rcing are used as a framework to discuss the shortest periods of chang e 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 oc ean, we show that the geochemical response to changing fluxes is stron gly attenuated at periods shorter than the residence time, and that th e response at these periods is also phase shifted so as to lag the for cing by one quarter cycle. The amplitude response as a function of per iod is used to estimate the periods of forcing resolvable by present a nalytical precision in measuring the Sr-87/Sr-86 and U-234/U-238 evolu tion 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 co re. At first sight this correlation might be taken as good evidence th at both Sr-87/Sr-86 and deltaO-18 are responding to some common climat ic forcing, but given the long residence time of Sr in the ocean, Sr a nd O should in fact have different phase shifts relative to any propos ed common forcing. Thus the apparent correlation between Sr-87/Sr-86 a nd deltaO-18 over the past several 100 Ma is not necessarily an indica tion that they are responding to a common cause. Amplitude response cu rves are also used to place limits on permissible global seawater U-23 4/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.