Robust Beach Ridge Evidence Indicates Mid Holocene Sea Levels Were 1-5 Meters Higher Than Today

Three new studies use evidence from elevated beach ridges to assess sea levels were 3-5 m higher than today near Thailand, 1-1.5 m higher near Australia, 2-5 m higher along the North Sea just a few millennia ago.

The higher sea levels in the tropical regions (Australia, Gulf of Thailand) were “primarily driven by eustatic processes,” not a result of vertical land motions.

This means the Mid to Late Holocene highstands were mostly a consequence of the larger volume of water in ocean basins at that time relative to today, when there is more water locked up on land as ice.

Terry et al., 2024 (Gulf of Thailand)

“At an elevation of 3.3–5.3 m above modern sea level, the sequence is interpreted to represent a Holocene raised beach. The unlithified sediments comprise rounded quartz and mylonite pebbles and cobbles, oriented predominantly NE–SW, supported by fossiliferous sands that are rich in marine shells, coral fragments and occasional terrestrial gastropods. The juxtaposition of the marine and non-marine gastropoda of contemporaneous ages mC14 and OSL age-dating of shell material and mineral sands suggest the raised (storm) beach formed between 3.5 and 4.0 ka BP, i.e. ∼ 2.5–3.0 ka after the MHH peak, at a height of ∼ 1.3–3.3 m above the local RSL position at that time (according to glacial isostatic adjustment modelling). Given the otherwise paucity of data from the upper GoT, the Ko Khang Khao raised beach provides nakes a compelling story for a coastal storm deposit, thrown up either by a winter monsoon storm, or by a palaeotyphoon that managed to penetrate the upper Gulf. Overlapping results of ew information that expands our current understanding of geographical variations in RSL across Southeast Asia during the Late Holocene.”

 Image Source: Terry et al., 2024

Kennedy et al., 2024 (Australia)

“Holocene sea-level in Victoria is considered to be primarily driven by eustatic processes and is most likely similar to that of eastern Australia broadly (Bryant, 1992; Kennedy et al., 2020b; Lewis et al., 2013). A maximum elevation during the Holocene occurred at around +1.0–1.5 m 6000–6500 calibrated years ago (cal BP) has been suggested based on dating of raised notches (Cape Liptrap; Baker et al., 2001; Gardner et al., 2009) and estuarine sediments (Anglesea; Kennedy et al., 2021) in southern Victoria  … As the estuary closely tracked sea level during the mid-Holocene it can therefore be used to provide further indication on the spatial extent of the maximum sea level elevation recorded elsewhere in eastern Australia (Lewis et al., 2013) and on the other side of the Tasman Sea (Clement et al., 2016; Kennedy, 2008; Kennedy et al., 2022) during this period. Evidence for a mid-Holocene sea level highstand along the open coast of Victoria, southern Australia, has generally been scant. Fixed biological indicators at Cape Liptrap (165 km east-south-east) suggest a higher sea level of 1.5 m at around 5.5–5.0 ka (Gardner et al., 2009; Haworth et al., 2002), while at Anglesea elevated estuarine shells at +1.14 m date at 6704–6381 cal BP (Kennedy et al., 2021; Figure 9). As the average elevation of the infilled part of the estuary is +2.34 ± 0.51 m and composed of estuarine sediments this further suggests deposition during a period of higher sea level. Combined with the shallowest age (1 m from the surface, +1.021 m above MSL) within Aireys/Painkalac of 6930–6440 year cal BP being very similar to that of Anglesea provides further evidence that a period of higher sea level during the mid-Holocene occurs along the south east coast of Australia.”

Image Source: Kennedy et al., 2024

Freiesleben et al., 2024 (North Sea)

“The postglacial sea-level history in the Limfjord area is based on radiocarbon dating of terrestrial-to-marine transitions in sediment cores, and mollusc shells in raised marine deposits showing a rapid sea-level rise in the Early Holocene (Petersen 1979; Bennike et al. 2019; Jessen et al. 2019) followed by a Middle Holocene peak in sea level 2–5 m higher than present as documented by the raised beach ridges and Littorina shorelines (Mertz 1924).”

Image Source: Freiesleben et al., 2024