How do modern winter water temperatures in the southern Gulf of California compare with winter water temperatures during the last interglacial, a time period thought to be slightly warmer than today. Modern and fossil corals from the southern Gulf of California may provide the answer. Previous studies have suggested that winter temperature is the controlling factor limiting the growth of Porites. Today, the northernmost Porites reefs in the Gulf of California occur near the southern tip of Baja California at Cabo Pulmo (modern winter isotherm ~20 C). In contrast, during the last interglacial (MIS 5e. ~125,000 ybp), this same species of coral grew in reefs approximately 400 km further north near the town of Loreto (modern winter isotherm ~17 C). These data suggest that winter water temperatures near Loreto were ~3 C warmer during the last interglacial. Since stable oxygen isotope ratios (d18O) in biogenic carbonates vary in part as a function of temperature, winter d18O values from modern and Pleistocene corals can be used to test this hypothesis. Thus, modern and fossil Porites from the Gulf of California provide a unique opportunity to evaluate regional climate change over the last 125,000 years.

Samples of Pleistocene corals were collected at Punta Bajo near Loreto. These samples were cut parallel to the axis of maximum growth and sampled for isotopes using a microdrill. 12-15 samples were taken from each year, providing approximately monthly resolution. The average maximum d18O value (winter) is -2.7 permil. Using a Porites paleotemperature equation, we calculated water temperatures at the site of the northern most Porites-reefs from the last interglacial. However, prior to calculating paleotemperatures, we corrected for differences in the d18O of the ocean associated with changes in the sizes of polar ice-caps and the resulting rise in sea-level. During the last interglacial, sea-level was ~6 m higher than today. Assuming a 0.01 permil difference for every 1 m in sea level change, we corrected coral d18O values by 0.06 permil. Because, in addition to temperature, the d18O in biogenic carbonates is a function of the isotopic composition of the water in which they grew, we also need to estimate the d18O of the southern Gulf of California during the last interglacial. Previous studies assume that the modern d18O value of surface water in the Guaymas basin is ~0.36 permil. We assume the same value for regional surface water during the last interglacial. The reconstructed paleotemperatures indicate that the average winter water temperature was ~20.5 C. Today, the winter water temperature at Punta Bajo is on average ~17.7 C. This indicates that the winter temperature during the Pleistocene were ~2.8 C warmer than today. Furthermore, the reconstructed winter paleotemperature at Punta Bajo (~20.5 C) agrees well with the winter temperature at the northern limit of modern reefs in the Gulf of California (~20 C). These data suggest that warmer winter temperatures in the southern Gulf of California may have facilitated the northward expansion of Poritres-reef during the last interglacial.

The results of this geochemical analysis may be evaluated through further investigation of annual growth rates of modern and Pleistocene corals. Annual growth rates in Porites are a function of temperature and because density bands are deposited during the cold winter months, the distance between successive bands records the annual growth rate. Thus comparison of modern and Pleistocene growth rates may shed additional light on relative paleotemperatures during the last interglacial.

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