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The Potential Role of the Evolving South China Sea Tidal System on the Size of Mangrove Biome and Associated Hydrocarbon Source Material During the Tertiary


Mangroves are highly productive low latitude coastal ecosystems. They play a crucial role in the modern marine carbon cycle and in the geological past, the remains of mangrove plants have formed coals and sourced substantial hydrocarbon resources. The areal extent of mangroves is limited by tidal range which defines the habitable extent of mangroves relative to competitive freshwater species. Tidal activity can thus be considered a first order control on the carbon capture capacity for the mangrove system. Attenuation of tidal flow by dense root networks of mangrove plants promotes sedimentation of fine-grained sediment. This promotes burial and sequestration of the biological carbon in the lithosphere. Herein we use Fluidity, an unstructured mesh finite element ocean model, to simulate the evolution of the tidal system in the South China Sea over the Chattian-Messinian. The changing tidal dynamics of the area result from: an evolution from a blind gulf to open seaway; basin orientation relative to the paleo-Pacific; and the relative position of the Philippines. Across a range of sensitivity tests, modelled tidal range along palaeo-coastlines were higher (meso-macrotidal) relative to the present day (micro-mesotidal). Higher tidal ranges would have resulted in a wider marine-influenced zone for mangrove production. Model output includes bed shear stress which is an indicator of the sediment transport capacity for the tides. At times (Chattian-Serravalian) over the period modelled, coupling of tidal range and bed shear stress occurred as the low amplitude tidal wave shoaled over the shallow shelf. At such times the burial efficiency of the mangrove biome may have been especially enhanced. Fossil mangrove and mangrove-related facies have been recognised in the Miocene stratigraphic record of onshore Borneo (Sarawak, Brunei and Kalimantan). TOC values of mangrove sediments in the modern Mahakam Delta range from 4-12%. Similar carbon-rich mangrove facies buried in offshore northwest Borneo act as both effective sources and seals in tide-dominated reservoirs facies in the prolific, hydrocarbon-bearing Baram Delta Province. Integrating facies analysis and paleotidal modelling therefore provides predictive insight into the potential for effective carbon storage and sequestration in mangrove-related coastal facies. This integrative analysis may in turn permit semi-quantitative analysis of the effect of mangrove biome changes on global carbon-cycle dynamics.