Records and causes of Holocene salinity shifts in Laguna de Bay, Philippines

Abstract

A 7000-year record of salinity shifts from molluscan and diatom assemblages and verified geochemical salinity proxies (C/S, S, Sr, Ca, and Sr/Ba) is reconstructed from Philippines' largest freshwater lake Laguna de Bay. The salinity shifts are interplays of millennial-scale sea level change, centennial-scale tectonic activity from the West Marikina Valley Fault (WMVF) and decadal-scale climatic changes. Currently only 1 m above mean sea level, the lake is connected to Manila Bay through the Pasig-Napindan River, which meanders across a densely populated “Parañaque Strip” partly occupied by metropolitan Manila. The controversial WMVF borders the lake on its western shore. Our 10.5 m sediment core from the western lobe reveal that the deposits are from a marine environment deposited 6600 cal BP to 4700 cal BP during a sea level still stand above present mean sea level (apmsl) in Southeast Asia and the Philippines. Episodic decreases in salinity with 260 y cycles were traced to the activity of the WMVF that would have accounted for at least 6 m of uplift of the Parañaque Strip. The vertical component of movement raised a sill and emergent landmass that started to isolate the Paleo-Laguna de Bay from Manila Bay. Subsequent episodic uplifts and sea level fall decreased depths to intertidal levels by 4700 cal BP, salinity to brackish but highly-influenced by marine waters by 4100 cal BP and to freshwater by 3100 cal BP. The latest salinity increase since mid-1960 is attributed to renewed saltwater incursion due to abnormally dry years, low lake levels, and Pasig River delta plain subsidence.