Understanding the precise phase relationships of changes in sea level, temperature, and greenhouse gas concentrations during previous interglacial periods provides us with critical knowledge to evaluate the future response of the climate system to anthropogenic forcing. Absolutely-dated sea level archives that document the response of ice sheets to changes in temperature and atmospheric CO2 become increasingly rare as we look beyond the last interglacial due to the combined effects of alteration, physical superposition of multiple sea level oscillations, and challenges related to temporal limitations of geochronometers.
We have studied a suite of submerged stalagmites from Argentarola Cave, Italy collected across a depth range of -18 to -21 m to improve the absolute chronology of several sea level highstands during MIS 7, also referred to as the penultimate interglacial (Fig. 1). The spectacular feature of speleothems recovered from Argentarola Cave is the occurrence of alternating layers of biogenic calcite and spelean calcite that result from multiple sea level oscillations in the past. The biogenic layers are composed of serpulid calcite secretions that encase the speleothems during seawater submergence while the spelean calcite growth only occurs when the cave is emergent, or above sea level. We have used U-series dating techniques to determine the precise timing of speleothem growth that brackets each serpulid calcite layer to ascribe a chronology to three sea level highstands during the penultimate interglacial. The second highstand, also known as marine isotope stage (MIS) 7.3, is observed to peak at a lower elevation than the other two highstands and appears delayed in timing relative to peak northern hemisphere insolation. We postulate that this behaviour results from the intense cold period preceding MIS 7.3 that was associated with significant development of ice sheets in the northern hemisphere.
Our findings underpin the importance of cryosphere state as a critical factor determining the sensitivity of sea level response to insolation forcing. While records from submerged speleothems such as this are rare, they are archives that have enormous potential to shed light on the dynamics of climate and sea level in the past, and also to inform us about the interplay of these variables as we head into the future.
More details can be found at Nature Geoscience.