Evaluating late Holocene relative sea-level changes from the tropics: Matang Mangrove Forest Reserve, Malaysia.

Reconstructions of past relative sea level (RSL) during the late Holocene have shown a response to natural climate warming and cooling phases such as the Medieval Climate Anomaly and Little Ice Age. Coupled with long-term instrumental measurements from tide gauges, they also showed a timing of emergence in RSL rate centered around the mid-19th century, with a 20th century rise that is extremely likely (P≥0.999) faster than the proceeding 3000 years. These conclusions, however, are derived from RSL reconstructions and tide-gauge records that are limited in tropical latitudes and currently excludes Southeast Asia hindering the interpretation of sea-level changes and validation of models that predict future spatial variability.

Here, we present a new RSL reconstruction using mangrove sediments from the Matang Mangrove Forest Reserve, western Peninsula Malaysia to constrain RSL change during the late Holocene. Following an extensive field reconnaissance, we collected a ~3 m core from the upper intertidal environment and modern surface samples across an intertidal-to-mangrove gradient to constrain modern and fossil indicative meanings. Selected samples were analyzed for organic content, foraminiferal assemblages and environmental DNA and sample sites were surveyed relative to local Malaysian national geodetic benchmarks using differential GPS. We constrained temporal uncertainties in the reconstruction using accelerator mass spectrometry radiocarbon dating of bulk sediment fine-fractions (n=11) coupled with short-lived radionuclide chronohorizons within a Bayesian age-depth framework.

Stratigraphic investigations revealed uniform sedimentary sequences comprising subtidal and intertidal silty clay muds overlain by organic (50% LOI) mangrove peats to depths of ~2.5 m within which foraminiferal tests are well preserved and dominated by typical agglutinated taxa including Arenoparrella mexicana and Trochammina inflata. Radiocarbon dating provides an excellent chronology of in sequence ages approximately ~2200 years old. We combined the proxy reconstruction with nearby tide gauge records and applied a spatiotemporal empirical hierarchical model to quantify magnitudes and rates of RSL change. We compare the RSL reconstruction with other new records from Singapore and use glacial isostatic adjustment model predictions to assess and discuss driving processes throughout the region.