TERTIARY PALEOCEANIC CHEMICAL VARIABILITY: UNINTENDED CONSEQUENCES OF SIMPLE GEOCHEMICAL MODELS

Five lines of concrete, quantitative evidence about oceanic chemical composition during the past 65 m.y. can provide insights into the controlling processes and their temporal variations, but the system constraints are not yet fully understood. The calcium carbonate compensation depth, mean carbon isotopic ratio, and strontium isotopic ratio have all changed significantly during this time, while changes in lithium-to-calcium and strontium-to-calcium ratios have been much smaller. When these tracers are considered separately, simple scenarios of changes in geochemical and geological factors can be used to explain the historical records. These factors include: (1) increases in continental chemical weathering rates and net continental fluxes to the oceans, (2) changes in the isotopic composition of material weathered from continents to the oceans, (3) decreases in the proportion of calcium carbonate deposited on continental shelves, (4) changes in the deposition or weathering rates of certain types of sediments, and (5) changes in hydrothermal circulation. Because these geochemical cycles are linked to each other (and to others, such as nutrients and global atmospheric carbon dioxide), an explanation invoked for one tracer must satisfy the direction, magnitude, and timing of the patterns for all the tracers. Several simple scenarios for geochemical variations have been evaluated for their consequences (both intended and unintended) and compared to the historical records. Despite the attractiveness and possible justifications of some simple hypotheses, for example, decreases in organic carbon burial rates, increases in carbonate weathering rates on the continents, or increases in all continental weathering rates, no one simple scenario appears to be consistent with both the timing and magnitude of the changes observed during the past 65 m.y. for all tracers considered. The contradictions uncovered can be useful in defining what knowledge is crucial to further understanding of oceanic chemical history. Better knowledge is needed of absolute deep-ocean sediment accumulation rates and their temporal variability, of the extent and time scale of variability in the net carbon isotope input to the ocean from continental weathering, of steady state fluxes for oceanic mass balances and the extent to which these mass balances approach steady state behavior, and of the links and interactions between the geochemical cycles of various elements.