Frank M. Richter

Sr isotope evolution of seawater: the role of tectonics

We use a high-resolution seawater Sr isotopic evolution curve for the last 100 m.y. in conjunction with modern riverine Sr flux measurements, and also geologic, tectonic and geochronological data, to make the case for a close relationship between seawater Sr isotopic composition and the India Asia continental collision. Using a simple seawater Sr budget model we begin by showing that the Sr flux associated with alteration of seafloor basalts is too small and does not have the right time evolution to account for much of the seawater Sr isotopic curve of the last 100 m.y. The flux of dissolved Sr carried by rivers originating in the Himalaya-Tibet region on the other hand is presently a significant fraction of the global Sr budget. We calculate how this riverine flux would have had to change with time in order to match the observed seawater Sr isotopic curve and find that the riverine flux remains relatively constant prior to the collision of India with Asia but then increases very significantly after collision. We note that the period of most rapid change in seawater Sr isotopic ratio, from 20 Ma to 15 Ma, is also a period of exceptionally high erosion in parts of the Himalayas and the Tibetan Plateau. As further evidence that Sr derived from the collision of India with Asia plays a major role in the Sr isotopic evolution of seawater we show that the total amount erosion of the Himalaya-Tibetan Plateau since collision, which we calculate separately, represents a total amount of Sr that is very nearly the same as the cumulative amount required by the Sr isotopic change of seawater since collision. The relationship between erosion and riverine Sr flux allows us to use the Sr isotopic evolution of seawater to reconstruct a history of erosion since collision, and we find that the erosion rate accelerates with time since collision, with the present having the largest rate. When we apply the Sr budget model to the entire Phanerozoic using a new compilation of deformed continental area versus time we find that we can account for the large-scale structure of the seawater Sr isotopic curve, but fail to reproduce several local maxima and minima, especially in the period 100-300 Ma. The present high S7Sr/S6Sr of seawater and similar highs in the Devonian and Cambrian do correlate with extensive deformation on the continents.

Dynamical Models for Melt Segregation from a Deformable Matrix

A mathematical formulation for the buoyancy-driven segregation of a light fluid from a deformable matrix is applied to a geometrically simple system to illustrate its dynamical and chemical evolution. A layer with initially uniform fluid content contained above and below by impermeable boundaries, if sufficiently deep, displays three regimes: a compacting boundary layer at its base, a non-compacting interior regime, and a growing pure fluid layer at the top. Various other initial conditions are discussed, including cases with linearly decreasing fluid content and with locally large (in relative terms) fluid content. The chemical evolution of the segregating fluid is also described. Having illustrated some of the properties of fluid segregation by matrix compaction, we use it as a framework for discussing the secular changes in

Diffusion domains determined by 39Ar released during step heating

^{143}Nd/^{144}Nd

A relation between the driving force and geoid anomaly associated with mid-ocean ridges

and Sm/Nd of Hawaiian basalts.

Nonlinear waves in compacting media

A numerical model which describes oxygen isotope exchange during burial and recrystallization of deep-sea carbonate is used to obtain information on how sea surface temperatures have varied in the past by correcting measured δ18O values of bulk carbonate for diagenetic overprinting. Comparison of bulk carbonate and planktonic foraminiferal δ18O records from ODP site 677A indicates that the oxygen isotopic composition of bulk carbonate does reflect changes in sea surface temperature and δ18O. At ODP Site 690, we calculate that diagenetic effects are small, and that both bulk carbonate and planktonic foraminiferal δ18O records accurately reflect Paleogene warming of high latitude surface oceans, biased from diagenesis by no more than 1°C. The same is likely to be true for other high latitude sites where sedimentation rates are low. At DSDP sites 516 and 525, the effects of diagenesis are more significant. Measured δ18O values of Eocene bulk carbonates are more than 2% lower at deeply buried site 516 than at site 525, consistent with the model prediction that the effects of diagenesis should be proportional to sedimentation rate. Model-corrections reconcile the differences in the data between the two sites; the resulting paleotemperature reconstruction indicates a 4°C cooling of mid-latitude surface oceans since the Eocene. At low latitudes, the contrast in temperature between the ocean surface and bottom makes the carbonate δ180 values particularly sensitive to diagenetic effects; most of the observed variations in measured δ18O values are accounted for by diagenetic effects rather than by sea surface temperature variations. We show that the data are consistent with constant equatorial sea surface temperatures through most of the Cenozoic, with the possible exception of the early Eocene, when slightly higher temperatures are indicated. We suggest that the lower equatorial sea surface temperatures for the Eocene and Oligocene reported in other oxygen isotope studies are artifacts of diagenetic recrystallization, and that it is impossible to reconstruct accurately equatorial sea surface temperatures without explicitly accounting for diagenetic overprinting.

Simple models for the geochemical response of the ocean to climatic and tectonic forcing

The Arrhenius plot for 39Ar released during step heating of a sample containing a distribution of diffusion domains will depart from a straight line in ways that depend on the domain size distribution and the heating schedule used to extract the 39Ar. In a new series of experiments we make use of the heating schedule effect to confirm the multidomain nature of several alkali feldspar samples. We find that temperature cycling is particularly useful for determining the diffusion parameters (activation energy and frequency factor) of the different domains. We also use the log (r/r0) versus cumulative percent 39Ar released plot presented in an earlier paper to display better the sizes of the different domains and the proportion of argon that they contain. We conclude by showing that a particular choice of the geometry and number of domains is not critical to the determination of the thermal history provided the 39Ar data are satisfactorily fitted by the particular choice mode. Experiments on single crystals show that multiple diffusion domains are an intrinsic property of the feldspars and thus are not separable by a careful selection of the grains to be analyzed.

Numerical models for diagenesis and the Neogene Sr isotopic evolution of seawater from DSDP Site 590B

Experiments with fluids whose viscosity depends strongly on temperature are used to study the effect of viscosity variations in the range 10−10 5 on the heat transfer and horizontally averaged temperature of a convecting layer between horizontal isothermal boundaries. At large viscosity variations (3 × 10 3 and 10 5 ) and Rayleigh numbers less than the critical value given by linear theory, the system can be either conductive or convective depending on whether the Rayleigh number is increased from an earlier conductive state or decreased from a preexisting convective state. At higher Rayleigh numbers and for the entire range of viscosity variation studied the heat transfer differs little ( R a / R c ) is even more remarkable being independent of viscosity variation and indistinguishable from that of a uniform-viscosity fluid with appropriate Prandtl number. The horizontally averaged temperature becomes increasingly asymmetrical with increasing viscosity variation due to the relatively large temperature change across the cold, more-viscous boundary layer, and results in an isothermal interior temperature significantly hotter than the average of the boundary temperatures. The measured temperature and convective heat transfer as a function of depth show that for viscosity variations greater than about 100 most of the viscosity change occurs within a stagnant conductive layer that develops above the actively convecting part of the system.

Models for the Archean thermal regime

The driving force and geoid anomaly associated with the thermal structure of the oceanic plates are shown to be proportional to the first moment of the density structure with respect to depth and, hence, to each other. Both quantities exhibit the same functional dependence on age and this is given for two different thermal models. For the plate model the geoid anomaly and ridge driving force only increase slowly for ages greater than 40 m.y. in contrast to the half-space boundary layer model where a linear dependence on age holds for all ages. Isolation of the geoid anomaly related to the thermal structure of the plates would provide a direct measure of the magnitude of the ridge driving force.