A sedimentary and foraminiferal record of early to mid-Holocene environmental change from Singapore

In the context of warming climate and rising sea level, records from the early-to-mid Holocene provide important analogues to investigate how the environment responds to such changes. The Sunda shelf provides favourable conditions to reconstruct past environmental change as the presence of numerous large paleo-valley systems and high sedimentation rates allow transgressive deposits from the early-mid Holocene sea-level rise to accumulate continuously in topographic depressions.  To this end, we analysed the sedimentological, geochemical and micropaleontological characteristics of a sediment core (GRBH03) to investigate early-to-mid Holocene environmental changes in southern Singapore. We constrained the chronology with ten radiocarbon dates that were placed in Bchron age-depth model. Using a multi-proxy approach (e.g., grain size distribution, loss on ignition and XRF core-scanning), supported by benthic foraminifera, three sedimentary units were identified in GRBH03. Sedimentary unit I was found at the base of the core. This unit was characterised by a dark grey sandy silt unit deposited from about 9.0 to 8.9 cal ka BP. Few or no foraminifera were found in this unit, likely due to degradation of organic material. Sedimentary unit II was a blue-grey marine mud that was deposited between 8.8 and 5.8 cal ka BP. Within the marine mud unit, foraminiferal assemblages show a transition to shallow marine environment from about 8.8 to 6.4 cal ka BP as open marine species (e.g., Murrayinella murrayi and Bulimina sp. cf. B. marginata) become more abundant up-core. Subsequently, brackish species such as Muyrrayinella globosa and Ammonia veneta started to dominate mud unit assemblages, reflecting a transition from shallow marine to brackish environments, likely associated with decelerating sea level rise. The marine unit is then overlain by the sedimentary unit III, which is a shelly-silt unit deposited after 5.9 cal ka BP. This unit was mostly barren of foraminifera, which may be attributed to a high-energy marginal marine environment where conditions were unfavourable for foraminiferal preservation. Our study show that changes in sedimentary units and foraminiferal assemblages present in GRBH03 are linked to varying rate of relative sea-level rise during the early-to-mid Holocene.