ALBERT

Description: 〈span〉Plate tectonics, the principal vehicle for dissipation of planetary energy, is believed to buffer the δ〈sup〉18〈/sup〉O of seawater at its near-modern value of 0‰ SMOW (Standard Mean Ocean Water) because the hot and cold cells of hydrothermal circulation at oceanic ridges cancel each other. The persistence of plate tectonics over eons apparently favors attribution of the well-documented oxygen isotope secular trends for carbonates (cherts, phosphates) to progressively warmer oceans, from 40–70 °C in the early Paleozoic to 60–100 °C in the Archean. We argue that these oceanic hydrothermal systems are dominated by low-temperature (〈350 °C) cells that deplete the percolating water in 〈sup〉18〈/sup〉O. Seawater δ〈sup〉18〈/sup〉O is therefore a proxy for, rather than being buffered by, the intensity of plate tectonics. Detrending the Phanerozoic carbonate δ〈sup〉18〈/sup〉O〈sub〉c〈/sub〉 secular trend for its “tectonic” component yields a stationary time series that, interpreted as a proxy for Phanerozoic climate, indicates low-latitude shallow ocean temperatures oscillating between 10 and 30 °C around a baseline of 17 °C, attributes comparable to modern temperature values.〈/span〉

Description: 〈span〉Porewater extractions and acid leachates of rock core from a 250 m thick sequence of low-permeability Ordovician-age shales and limestones, on the eastern flank of the Michigan Basin, were analysed for strontium isotope ratios in an attempt to infer porewater ages from observed 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr enrichments. The porewaters originated as Ordovician seawater, which subsequently mixed with evaporated Silurian seawater infiltrating from above, and, to some extent, with a deep brine—with an enriched 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr signature—from the underlying crystalline shield or deep basin. The porewater 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr ratios are more radiogenic than contemporaneous seawater but show no obvious correlation to those leached from the solid rock phases. Accepting that the initial 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr signatures in porewaters were dominated by Late Silurian brine, potentially with an additional deep brine component, the excess of radiogenic 〈sup〉87〈/sup〉Sr appears to represent ingrowth from 〈sup〉87〈/sup〉Rb decay over a time span of some 420 million years, approaching the depositional age of the rocks. Similarly, Rb/Sr errochron ages of acid leachates of solid phases, and the calculated initial 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr isotopic ratios, are consistent with a proposition that the calcites inherited their Sr from Ordovician seawater and were dolomitized shortly afterwards by infiltrating Mg-enriched evaporative brine, indicating long-term conservative behaviour for the enclosing carbonate rocks. The errochron for leachates from (alumino)silicates yields a high initial 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr, but with an errochron age of about 340 ± 48 Ma, likely owing to variable admixtures of diagenetic illite in the shales. Overall, the data provide evidence for a stable hydrologic regime since Paleozoic time.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Plate tectonics, the principal vehicle for dissipation of planetary energy, is believed to buffer the δ〈sup〉18〈/sup〉O of seawater at its near-modern value of 0‰ SMOW (Standard Mean Ocean Water) because the hot and cold cells of hydrothermal circulation at oceanic ridges cancel each other. The persistence of plate tectonics over eons apparently favors attribution of the well-documented oxygen isotope secular trends for carbonates (cherts, phosphates) to progressively warmer oceans, from 40–70 °C in the early Paleozoic to 60–100 °C in the Archean. We argue that these oceanic hydrothermal systems are dominated by low-temperature (〈350 °C) cells that deplete the percolating water in 〈sup〉18〈/sup〉O. Seawater δ〈sup〉18〈/sup〉O is therefore a proxy for, rather than being buffered by, the intensity of plate tectonics. Detrending the Phanerozoic carbonate δ〈sup〉18〈/sup〉O〈sub〉c〈/sub〉 secular trend for its “tectonic” component yields a stationary time series that, interpreted as a proxy for Phanerozoic climate, indicates low-latitude shallow ocean temperatures oscillating between 10 and 30 °C around a baseline of 17 °C, attributes comparable to modern temperature values.〈/span〉