Science Diliman, Vol 17, No 2 (2005)

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Compaction rates and paleo-sea levels along the delta complex north of Manila Bay, Luzon Island, Philippines

Janneli Lea Soria, Fernando Siringan, Paul Erwen Parreño

Abstract


Uncontrolled groundwater extraction has been proposed as the main cause of accelerated subsidence in the delta region north of Manila Bay. However, natural autocompaction of deltaic sediment and other anthropogenic factors also enhance subsidence, amplifying global sea-level rise and aggravating land loss, flooding, and tidal inundation. Here, we report how we determine the longer-term subsidence rates and paleo-sealevel history of the delta plain using sediment cores. Four sediment cores 3 to 10.7 m long taken in Bocaue and Malolos, Bulacan and Lubao, Pampanga all display shoaling-upward sequences that consist of, from bottom to top: basal shallow-marine clays comprising nearly half of each core; mangrove peat; beach sand; fluvial sand and mud; and uppermost floodplain clays. Porosities of the deltaic sediments range from 0.3 to 0.8. Peat has the highest porosities, from 0.7 to 0.8. Calculations indicate about 2 to 6 m of compaction for the whole sediment sequence. Wood fragments at 7 m and 8.4 m depths in the shallow-marine section of the Pampanga core respectively yielded radiocarbon ages of 1800 ± 40 and 1730 ± 40 years. If around 1,000 years ago is when the surface 10 m of sediments started compacting, they would have done so at rates of 0.2 to 0.6 cm/y. Natural compaction in similar environments such as in Po Delta, Italy and Mississippi Delta are comparable, ranging from 0.09 to 0.37 cm/y. The small values acquired in this study imply that large human-induced components may account for as much as 97 percent of the subsidence in Pampanga.

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