Transformation of Laguna Lake from a marine arm of Manila Bay to a freshwater system

Abstract only.Evolution of Laguna Lake during the past 6,000 years is established using paleontologic, sedimentologic, and geochemical profiles of a 10.5-meter long sediment core from the western lobe. Marine molluscan and diatom assemblages reveal a lagoonal environment with depths initially greater than 10 m, 5,700 radiocarbon years ago (yBP), then it consistently shallowed to intertidal depths until 3,700 yBP. Tellina sp., Leptaxinus sp., and Anomalocardia sp. predominated the molluscan fauna. The marine Cyclotella stylorum, Thalassiosira eccentrica, and Thalassiosira lineata, comprise 80% of the diatom fauna. Brackish water condition was short-lived and it was succeeded by freshwater conditions set at the appearance of Vivipara burroughiana 3,000 yBP. The XRF-derived sulfur, strontium, and calcium concentrations and their aluminum-normalized trends used as geochemical proxy records, corroborate the changing salinity levels and the age boundaries set from the molluscs. Lowering of sea level from the mid Holocene high and vertical movements across the West Marikina Valley Fault, which presently bound the western edge of Laguna Lake, led to the emergence of the Muntinlupa-Paranaque stretch, isolating the proto-Laguna Lake from Manila Bay. Pulsed decreases of salinity to freshwater levels at 250-year intervals and the associated emergence of a 30 m-high barrier reveal a large tectonic influence as opposed to small climatic and sea level shifts within this period. In the last few decades, a new phase of environmental shift has been occurring in the lake. Global warming and land subsidence elevated the saltwater backflow into the lake, enough to leave an increasing salinity trend in the sediment record

Typhoon Haiyan Overwash Sediments From Leyte Gulf Coastlines Show Local Spatial Variations With Hybrid Storm and Tsunami Signatures

Marine inundation associated with the 5 to 8 m storm surge of Typhoon Haiyan in 2013 left overwash sediments inland on the coastal plains of the northwestern shores of Leyte Gulf, Philippines. The Haiyan overwash deposit provides a modern sedimentary record of storm surge deposition from a Category 5 landfalling typhoon. We studied overwash sediments at two locations that experienced similar storm surge conditions but represent contrasting sedimentological regimes, namely a siliciclastic coast and a mixed siliciclastic-carbonate coast. The contrasting local geology is significantly reflected in the differences in sediment grain size, composition and sorting at the two sites. The Haiyan overwash sediments are predominantly sand and silt and can be traced up to ~ 1.6 km inland, extending farther beyond the previously reported \u3c 300 m inland limit of sedimentation. Sites with similar geology, topographic relief, and overland flow conditions show significant spatial variability of sediment thickness and inland extent. We infer that other local factors such as small-scale variations in topography and the type of vegetation covermight influence the spatial distribution of overwash sediments. The Haiyan overwash deposits exhibit planar stratification, a coarsening upward sequence, a non-systematic landward fining trend, and a sharp depositional (rarely erosional) basal contact with the underlying substrate. Overall, the Haiyan deposits have sedimentologic and stratigraphic characteristics that show a hybrid signature common to both storm and tsunami deposits

Micropaleontology of the 2013 Typhoon Haiyan Overwash Sediments from the Leyte Gulf, Philippines

Coastal geologic records allow for the assessment of long-term patterns of tropical cyclone variability. However, the accuracy of geologic reconstructions of tropical cyclones is limited by the lack of modern analogues. We describe the microfossil (foraminifera and testate amoebae) assemblages contained within overwash sediments deposited by Typhoon Haiyan when it made landfall on the islands of Leyte and Samar in the Philippines on 7 November 2013 as a Category 5 super typhoon. The overwash sediments were transported up to 1.7 km inland at four study sites. The sediments consisted of light brown medium sand in a layer \u3c1 to 8 cm thick. We used Partitioning Around a Medoid (PAM) cluster analysis to identify lateral and vertical changes in the foraminiferal and testate amoebae data. The presence of intertidal and subtidal benthic, and planktic foraminifera that were variably unaltered and abraded identify the microfossil signature of the overwash sediments. Agglutinated mangrove foraminifera and testate amoebae were present within the overwash sediments at many locations and indicate terrestrial scouring by Haiyan\u27s storm surge. PAM cluster analysis subdivided the Haiyan microfossil dataset into two assemblages based on depositional environment: (1) a low-energy mixed-carbonate tidal flat located on Samar Island (Basey transect); and (2) a higher-energy clastic coastline near Tanauan on Leyte Island (Santa Cruz, Solano, and Magay transects). The assemblages and the taphonomy suggest a mixed provenance, including intertidal and subtidal sources, as well as a contribution of sediment sourced from deeper water and terrestrial environments. (C) 2016 Elsevier B.V. All rights reserved

Indicators and possible driving mechanisms of shifts in the position of Agusan river along the Butuan coastal plain

The Butuan coastal plain is a fan-delta formed by deposition of Agusan River sediments in the past 7,400 radiocarbon years (ky). Traces of paleochannels that scar the coastal plain, seen in satellite images, indicate past river positions. Immediately prior to the present, Masao River was the active distributary as suggested by its well-defined and still continuous paleochannel. Numerous sudden shifts of river position have likely originated in the San Vicente area, the apex region of the fan-delta. These shifts could have been self driven or induced by floods or tectonics. Sediment input from rivers along the eastern flank of the Butuan coastal plain forced a number of downstream shifts. The function of the Agusan Marsh in buffering flashy river discharges in the Butuan coastal plain is threatened by present-day elevated sediment loads of rivers upstream

Predominant Nearshore Sediment Dispersal Patterns in Manila Bay

Net nearshore sediment drift patterns in Manila Bay were determined by combining the coastal geomorphology depicted in 1 : 50,000scale topographic maps and Synthetic Aperture Radar (SAR) images, with changes in shoreline position and predominant longshore current directions derived from the interaction of locally generated waves and bay morphology.Manila Bay is fringed by a variety of coastal subenvironments that reflect changing balances of fluvial, wave, and tidal processes. Along the northern coast, a broad tidal-river delta plain stretching from Bataan to Bulacan indicates the importance of tides, where the lateral extent of tidal influences is amplified by the very gentle coastal gradients. In contrast, along the Cavite coast sandy strandplains, spits, and wave-dominated deltas attest to the geomorphic importance of waves that enter the bay from the South China Sea.The estimates of net sediment drift derived from geomorphological, shoreline-change, and meteorological information are generally in good agreement. Sediment drift directions are predominantly to the northeast along Cavite, to the northwest along Manila and Bulacan, and to the north along Bataan. Wave refraction and eddy formation at the tip of the Cavite Spit cause southwestward sediment drift along the coast from Zapote to Kawit. Geomorphology indicates that onshore-offshore sediment transport is probably more important than alongshore transport along the coast fronting the tidal delta plain of northern Manila Bay. Disagreements between the geomorphic-derived and predicted net sediment drift directions may be due to interactions of wave-generated longshore currents with wind- and tide-generated currents