Not a bathtub: A consideration of sea-level physics for archaeological models of human migration

Abstract

Accurately reconstructing past sea level is key to simulating potential migration pathways of ancient hominins, including early Homo sapiens. Models of ancient human migration events commonly construct estimates of paleoenvironments using the "bathtub" model, in which sea level is assumed to rise and fall according to a "eustatic" (global average) value over time. However, large uncertainties exist on past ice sheet sizes and shapes, particularly prior to the Last Glacial Maximum (LGM), ~26,000 years ago. Moreover, regional sea level varies significantly due to the effects of glacial isostatic adjustment (GIA). That process includes Earth's gravitational, deformational, and rotational response to changing surface (ice plus ocean) loads across the ice age. Here, we offer an updated account of the physics of GIA-induced sea-level change and consider the impacts of these effects, together with a newly published ice sheet history, on sea-level changes across the last glacial cycle. As illustrations, we highlight the significance of these issues for studies of ancient human migration from Sunda to Sahul and for the timing of the final, post-LGM flooding of the Strait of Dover. These examples demonstrate the importance of incorporating updated ice sheet histories and accurate sea-level physics into archaeological research.

Constraints on Marine Isotope Stage 3 and 5 Sea Level From the Flooding History of the Karimata Strait in Indonesia

“…Sea level induced exposure and inundation of the Sunda Shelf during MIS 3, 4, and 5 likely played an important role in atmospheric convection (Di Nezio et al., 2016; Pico et al., 2020) as well as the temperature profile of the ITF (Gordon et al., 2011). The earliest evidence of modern humans on the island of Borneo is dated to ∼40 ka, the middle of MIS 3 (Borregine et al., 2022; Clarkson et al., 2017; de Bruyn et al., 2014; Husson et al., 2020; Kealy et al., 2018; O’Connell et al., 2018; Teixeira et al., 2021) when our results show an exposed Sunda Shelf and Karimata Strait would have facilitated migration from the mainland.…”

“…We provide the most direct evidence that the Karimata Strait was exposed during the full duration of MIS 3 and could have served as a land bridge to facilitate land based human migration from Sumatra to Borneo. There is further evidence that modern humans reached the Sahul Shelf (between Australia and New Guinea) by ∼40 ka and the islands between the Sunda and Sahul Shelves (Wallacea) by ∼50–70 ka (Borregine et al., 2022; Kealy et al., 2018; O’Connell et al., 2018). Our data support the conclusion that these migrations were also likely facilitated by a Karimata Strait land bridge (Borregine et al., 2022; Kealy et al., 2018).…”

“…There is further evidence that modern humans reached the Sahul Shelf (between Australia and New Guinea) by ∼40 ka and the islands between the Sunda and Sahul Shelves (Wallacea) by ∼50–70 ka (Borregine et al., 2022; Kealy et al., 2018; O’Connell et al., 2018). Our data support the conclusion that these migrations were also likely facilitated by a Karimata Strait land bridge (Borregine et al., 2022; Kealy et al., 2018).…”

Sea-level rise in Southwest Greenland as a contributor to Viking abandonment

“…There is ample evidence for sea-level rise during and after the period of Viking occupation, and the close proximity of the settlements to the margin of the GrIS would have made them vulnerable to gradual flooding and loss of habitable land ( 34 ). In the vicinity of the Eastern Settlement, this evidence includes drowned ruins and sediment core analysis ( 35 ), a largely submerged warehouse close to Igaliku Church ( 36 ), and a drowned beach observed by side-scan sonar ( 26 ).…”

“…GIA encompasses the deformational, gravitational, and rotational Earth perturbations that arise from surface (ice and ocean) mass loading throughout glacial cycles. These effects can cause regional sea level (and therefore topography relative to sea surface height) to depart significantly from “bathtub” predictions of global mean sea level (GMSL) and have consequences for human migration research ( 34 , 41 ).…”