The last interglacial and mid-Holocene sea level variability in the northern Indian Ocean and the influence of hydro-isostasy

Abstract

Microtidal coastline around Sri Lanka, situated at the tectonically stable northern Indian Ocean, provides reliable evidence for sea level studies and to understand the hydro isostatic influence on eustatic changes. As the past sea level data are limited in the northern Indian Ocean region and due to the significant discrepancies among the outcomes of previous work, this study used stable, in situ sea level indicators to study the past sea level changes in the region.

Emerged and submerged in situ corals from both north (16 samples) and south (14 samples) were used as marine limiting points (MLP) while beach rocks (06 samples) were used as sea level index points (SLIPs) and lagoon sediment (01 samples) was used as terrestrial limiting point (TLP). Their elevations were measured precisely from the modern mean sea level. Age dating was carried out using Accelerator Mass Spectrometry (AMS), gas bench radiocarbon techniques, and the U-Th method.

Emerged corals at Punkuduthiviu (1.22 m), Iranathivu (0.82 m), and Kachchatheevu (3.55 m), islands located in the Palk Strait, record the evidence for Marine Isotope Stage (MIS) 5e (≊ 128 ka BP relative to 1950 AD) and MIS 5c (≊ 107 ka BP) sea level highstand in the northern Indian Ocean. Corals in northern Islands show that the mid-Holocene highstand (MHH) relative sea level (RSL) was >1.5 m in the north around 6500 cal yrs BP and started decreasing after that. When considering the eroded height and evidence from a wave-cut notch in Kachchatheevu, it can be inferred that the MHH RSL reached about 2 m. In contrast, RSL during MHH in the south reached its maximum (∼1.6 m) about 1000 yrs later (∼5500 cal yrs BP), as evidenced by corals and beach rocks. The post-glacial sea level reached the modern mean tidal level at the southern coast between 6250 and 6500 cal yrs BP and gradually decreased after the MHH. Most MLPs and SLIPs in the south are at lower elevations than in the north. The Holocene sea level variability in the north and south predicted by the ICE-6G_C Glacial Isostatic Adjustment model closely matches field data. The differences in magnitude and timing of MHH between north and south can be explained by contrasting local deflections in continental shelves caused by meltwater loading and transgression velocities, with the broad and gently sloping continental shelf in the north vs. the narrow and steeper shelf in the south.