Low-latitude Ordovician to Triassic brachiopod habitat temperatures (BHTs) determined from δ18O[brachiopod calcite]: A cold hard look at ice-house tropical oceans

Some researchers have suggested that Phanerozoic land-based ice sheets extended occasionally into the tropical realm. If true, the tropical ocean at those times must have been distinctly colder than at the Last Glacial Maximum (LGM) when northern hemisphere ice sheets did not extend below 38° latitude. Low-latitude ocean temperatures derived from oxygen isotopes of brachiopod shells test this hypothesis by comparing the temperature regime for Paleozoic and early Mesozoic low-latitude settings to the tropical temperature regime of the modern interstadial ocean, and to mean temperatures estimated for the tropical ocean at LGM. A running mean of pH-adjusted brachiopod habitat temperatures (BHTs) shows that Paleozoic low-latitude oceans were, on average, cool to cold relative to the modern interstadial tropical ocean. At times during Pennsylvanian, Serpukhovian, Tournaisian and Ordovician–Silurian glaciations, these tropical seas were indeed significantly colder on average than at the LGM. Ice-sheets within tropical latitudes can be reasonably predicted at those times. Abundant and diverse Paleozoic brachiopod communities reflect these cool tropical oceans, consistent with modern brachiopod ecological preference for colder waters. Amplified Paleozoic temperature oscillations suggest recurring global warming events which episodically drove these cold tropical oceans to temperatures significantly higher than the warmest modern tropical ocean.