To identify and quantify upwelling and freshwater influences in contrasting tropical ecosystems, we performed stable isotope analyses (δ 13 C and δ 18 O) on 13 serially-sampled modern Conus shells collected from coastal waters in the southwestern Caribbean (SWC, non-upwelling) and gulfs of Chiriquí (nonupwelling) and Panama (upwelling)

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[1] Records of seawater chemistry help constrain temporal variations in geochemical processes that impact the global carbon cycle and climate through Earth's history. Here we reconstruct Cenozoic seawater Sr/Ca (Sr/Ca sw ) using fossil Conus and turritellid gastropod Sr/Ca. Combined with an oxygen isotope paleotemperature record from the same samples, the gastropod record suggests that Sr/Ca sw was slightly higher in the Eocene ($11.4 AE 3 mmol/mol) than today ($8.54 mmol/mol) and remained relatively stable from the mid-to late Cenozoic. We compare our gastropod Cenozoic Sr/Ca sw record with a published turritellid gastropod Sr/Ca sw record and other published biogenic (benthic foraminifera, fossil fish teeth) and inorganic precipitate (calcite veins) Sr/Ca sw records. Once the uncertainties with our gastropod-derived Sr/Ca sw are taken into account the Sr/Ca sw record agrees reasonably well with biogenic Sr/Ca sw records. Assuming a seawater [Ca] history derived from marine evaporite inclusions, all biogenic-based Sr/Ca sw reconstructions imply decreasing seawater [Sr] through the Cenozoic, whereas the calcite vein Sr/Ca sw reconstruction implies increasing [Sr] through the Cenozoic. We apply a simple geochemical model to examine the implications of divergence among these seawater [Sr] reconstructions and suggest that the interpretation and uncertainties associated with the gastropod and calcite vein proxies need to be revisited. Used in conjunction with records of carbonate depositional fluxes, our favored seawater Sr/Ca scenarios point to a significant increase in the proportion of aragonite versus calcite deposition in shelf sediments from the Middle Miocene, coincident with the proliferation of coral reefs. We propose that this occurred at least 10 million years after the seawater Mg/Ca threshold was passed, and was instead aided by declining levels of atmospheric carbon dioxide.

Caribbean biota underwent major ecological and evolutionary transformation in the Pliocene–Pleistocene, but a lack of detailed paleoenvironmental reconstruction prevents thorough resolution of cause and effect. We quantify levels of upwelling and freshwater input into Caribbean coastal shelf ecosystems over the last ∼6 m.y. with >3300 stable isotope measurements from 74 fossil serially sampled gastropods by normalizing δ18O values to open-ocean δ18O from planktonic foraminifera. We find that the influence of Pacific-like upwelling in the southwestern Caribbean was low after 4.25 Ma but coastal ecosystems were heavily influenced by seasonal freshening until ca. 2.5 Ma, after which time low-freshwater conditions were established. The origination of modern oligotrophic coastal conditions was therefore a result of oceanographic change causing declining upwelling, and declining nutrients from terrestrial sources. We speculate that a southward shift of the Intertropical Convergence Zone, associated with Northern Hemisphere glaciation, reduced rainfall and terrestrial nutrient input and contributed to biotic turnover in the southwestern Caribbean, including the proliferation of modern reef communities.

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