Deep ocean δ18O seawater values in the middle Miocene: a new paradox?

Abstract:

New temperature records, based on Δ₄₇ compositions of benthic foraminifera from ODP Sites 761, 747 and 1171, indicate that deep waters in the Southern and Indian Oceans were very warm during the middle Miocene compared to modern. Combined with carbonate δ¹⁸O from the same samples, these temperatures translate into deep water δ¹⁸O values that are higher than expected for such warm climate states (~ 0.5 to 1‰ VPDB).

There are three primary mechanisms which possibly explain these relatively high values. The first relates to terrestrial storage of isotopically light water: after the expansion of the Antarctic ice sheet at the middle Miocene climate transition, our records imply greater than modern storage of isotopically light water on land. If this storage was in the form of expanded ice sheets, it would likely require an ice volume similar to the ice sheets of the last glacial maximum. However, a significantly larger ice volume compared to present is difficult to reconcile with the warm deep ocean temperatures reconstructed. A second possibility is that available temperature-δ¹⁸Oc equations inadequately describe δ¹⁸O fractionation in mid-Miocene benthic foraminiferal carbonate. This would be the case if vital effects or other controls (e.g., pH) on fractionation were different. A third possible explanation is that a saline deep water mass with high δ¹⁸O was present in the regions of the investigated sites. As deep water formation in low latitudes would be associated with higher temperatures than those observed, circulation to higher latitudes leading to refrigeration must be invoked for this to be a plausible mechanism. A final option is that diagenesis might have altered Δ₄₇ compositions, but agreement with Mg/Ca temperatures at Site 761 make this unlikely.

High seawater δ¹⁸O values have also been obtained elsewhere and with other proxies, thus evidence is mounting to suggest that these high values are accurate. Identical benthic carbonate δ¹⁸O records to those measured at our sites have been reported from deep sea locations around the globe, which suggests our results are widespread. To constrain the Miocene evolution of sea level and ice sheet response to warming, we need to better understand the character and cause of the high deep water δ¹⁸O that is now emerging from the next generation of temperature records.