A 4.2 million years record of interglacial paleoclimate from sclerochronological data of Florida carbonate platform (Early Pliocene to recent)

The climate of the Pliocene and early Pleistocene was in a transient mode from generally warmer climates of the early Neogene to the maximum glaciations of the late Quaternary. Increasingly severe coolings occurred episodically in the high latitudes, whereas the low latitudes remained warm. For the last 5 million years (Ma), rather constant sea surface temperatures have been recorded in the Western Atlantic warm pool; however, direct climate data on temperature and humidity from shallow near-shore settings are lacking so far. In this study we present a synthesis of 26 new and 46 (incl. 24 recent) published sclerochronogical stable isotope records (¹⁸O/¹⁶O, ¹³C/¹²C) with a sub-annual resolution from reef corals (z-corals) and mollusks. The fossils were sampled from shallow-water carbonate deposits of the Florida carbonate platform and belong to 12 interglacial time-slices spanning collectively the period from the Early Pliocene to the recent (4.2 to 0Ma). Although platform carbonates are believed to undergo rapid diagenetic stabilization due to the dissolution of metastable aragonite shells, we show that there is still a wealth of material to be recovered for large-scale systematic geochemical studies. We rule out significant diagenetic modifications of the stable isotope data because measured ¹⁸O/¹⁶O ratios from z-corals and mollusks converted into temperature give consistent results. Accordingly, annual mean temperatures have risen during the last 4.2Ma from ~23°C to 26°C in open waters, given the modern seawater value of ¹⁸O/¹⁶O is valid for Neogene. However, the global water value has changed due to long-term increases in ice volume even during interglacials, equivalent with a 2.3°C temperature rise. A net 5.3°C temperature increase over the last 4.2Ma is inconsistent with the deep-sea record, however, and suggestive of an overall increase of humidity effects in measured ¹⁸O/¹⁶O instead. Particularly cool temperatures have been registered at 1.9 and 2.5Ma, but combined ¹⁸O/¹⁶O and ¹³C/¹²C data identify these as artifacts from intensified evaporation which fits the overall restricted marine aspect of the fossil fauna in these time-slices. Seasonal temperature contrasts seem to have been high within restricted settings (~11°C) and small in mixed open marine units (7-8°C). Although this finding fits the modern situation with coastal environments undergoing 14°C seasonal change and the reef tract 7-9°C only, circumstantial evidence suggests reconstructions to be biased by sub-annual changes in the local seawater value for ¹⁸O/¹⁶O. Since ¹⁸O/¹⁶O seasonality has increased over the last 4.2Ma, we suggest the humidity of modern Florida to have evolved from dryer precursor climates of past interglacials, whereas temperatures in essence remained the same. This trend possibly represents the expression of the growing relevance of the Bermudas High pressure cell. Glacial climates of Florida cannot be reconstructed using our methodology as the Florida platform was emergent during sea level lowstands.