Cenozoic sea-level and cryospheric evolution from deep-sea geochemical and continental margin records

Abstract

Using Pacific benthic foraminiferal δ18O and Mg/Ca records, we derive a Cenozoic (66 Ma) global mean sea level (GMSL) estimate that records evolution from an ice-free Early Eocene to Quaternary bipolar ice sheets. These GMSL estimates are statistically similar to “backstripped” estimates from continental margins accounting for compaction, loading, and thermal subsidence. Peak warmth, elevated GMSL, high CO2, and ice-free “Hothouse” conditions (56 to 48 Ma) were followed by “Cool Greenhouse” (48 to 34 Ma) ice sheets (10 to 30 m changes). Continental-scale ice sheets (“Icehouse”) began ~34 Ma (>50 m changes), permanent East Antarctic ice sheets at 12.8 Ma, and bipolar glaciation at 2.5 Ma. The largest GMSL fall (27 to 20 ka; ~130 m) was followed by a >40 mm/yr rise (19 to 10 ka), a slowing (10 to 2 ka), and a stillstand until ~1900 CE, when rates began to rise. High long-term CO2 caused warm climates and high sea levels, with sea-level variability dominated by periodic Milankovitch cycles.

Ancient Sea Level as Key to the Future

“…Ancient climates range from cold, glacial periods (e.g., Last Glacial Maximum [LGM]) with CO 2 at 180 ppm to warm, ice-free states, the most recent of which are the Early Eocene Climatic Optimum (EECO; 56-48 Ma) and possibly the Miocene Climatic Optimum (17-13.8 Ma), with CO 2 two to three times higher than 1850 CE. Miller et al (2020) recognize three pre-Anthropocene climate states: Hothouse (very warm, largely ice-free conditions; Late Cretaceous and Early Eocene), cool Greenhouse (Early to Middle Eocene) with small ice sheets (<25 m sea level equivalent), and Icehouse conditions with continental-scale ice sheets at one or both poles (Figure 2). Today, the Greenland Ice Sheet (GIS) contains 7.4 m of sea level equivalent; the West Antarctic Ice Sheet (WAIS) contains 5.6 m of sea level equivalent, including the Antarctic Peninsula; the East Antarctic Ice Sheet (EAIS) contains 52 m of sea level equivalent; and mountain glaciers and ice caps contain <1 m of sea level equivalent (Morlighem et al, 2019).…”

“…13.8 Ma), and large Northern Hemisphere ice sheets developing in the Quaternary (last 2.55 million years). Changes in ice volume dominate the rise and fall of sea level during Icehouse, cool Greenhouse, and perhaps even Hothouse worlds (Miller et al, 2020).…”

“…For the Late Pleistocene to present (last 129 kyr), dating of corals or marshes formed near sea level provides the most accurate means or reconstructing sea level. Measurements of foraminifera δ 18 O records reflect ice volume and temperature changes that can be calibrated to sea level by independent temperature estimates back through the Middle Eocene (e.g., Miller et al, 2020). Flooding (transgressions) and exposure (regressions) of the continents reflect GMSL and tectonic processes, as does the record of sequences (unconformity bounded units; Vail et al, 1977).…”

“…Drilling by the International Ocean Discovery Program (IODP) and its predecessors has provided material for independently estimating sea level using sequence stratigraphy and backstripping (progressively accounting for the effects of compaction, loading, and thermal subsidence; John et al, 2004;Miller et al, 2005a), and for combining deep-sea benthic foraminiferal δ 18 O and Mg/Ca records, with the latter providing an independent paleothermometer (Lear et al, 2000;Cramer et al, 2011;Miller et al, 2020). Convergence of these two methods ( Figure 2B) allows identification of GMSL changes and inferences about their mechanisms (Miller et al, 2020). In addition, coral drilling in Barbados ( Figure 3A; Fairbanks, 1989) and Tahiti by IODP Expedition 310 (Deschamps et al, 2012) provides constraints on the rates of sea level rise during the last deglaciation.…”

“…Finally, studies of Holocene marshes have produced pristine chronologies and estimates of the rates of GMSL rise during the Holocene (Figure 3B), including the CE, that can then be linked to instrument records (tide gauge and satellites). Here, we provide a geological perspective on past, present, and future sea level change using published backstripped (Miller et al, 2020), δ 18 O-Mg/Ca (Miller et al, 2020), coral (Peltier and Fairbanks, 2006;Deschamps et al, 2012), marsh (e.g., Kemp et al, 2009Kemp et al, , 2018Kopp et al, 2016;Horton et al, 2018), and instrumental (Dangendorf et al, 2017) sea level records.…”