Geochemical proxy records to reconstruct terrestrial input, redox conditions, and biological production in a tropical paleo-lagoon during the mid-Holocene

At the forefront of Asian and Pacific climate phenomena, Laguna de Bai in the Philippines could fill knowledge gaps in tropical Western Pacific paleoenvironmental records and help to forecast possible scenarios for semi-enclosed coastal water bodies under the predicted warmer and wetter climate with sea level rise by the end of the 21st century. Using geochemical proxy and grain size records along a sediment core, we describe three major marine phases from 6,510 to 4,590 years ago, when Laguna de Bai's Western Lobe was part of Manila Bay that slowly separated due to the tectonic uplift of the Parañaque Land Strip and interaction with sea level changes during the Holocene. Compared to present-day conditions, the slightly drier Coastal Sub-Tidal Lagoon Phase (6,510 to 6,260 cal BP) had usual marine salinity, oxic bottom waters, and only a partially preserved biological production record, with rainfall similar to that of the early 20th century. The episodic uplift of Parañaque Land Strip and drying climate transformed the Western Lobe into a Shallower Coastal Lagoon (6,260 to 5,310 cal BP) with higher bottom water salinity, suboxic bottom waters, and better preservation of biological production, under an increasingly drier climate. Further uplift of the Parañaque Land Strip restricted Laguna de Bai's Western Lobe even more and commenced the Intertidal Lagoon Phase (5,310 to 4,590 cal BP) with hypersaline, anoxic bottom waters, and well-preserved biological production record, all of which during the mid-Holocene dry period in the Western Pacific.