Massive corals record deforestation in Malaysian Borneo through sediments in river discharge

Abstract

Abstract. Logging of tropical primary forests is a widely acknowledged global issue threatening biodiversity hotspots and indigenous communities leading to significant land erosion and decreased soil stability. The downstream effects of logging on human coastal communities include poor water quality and increased sedimentation. Quantifying the impacts of historical deforestation within a watershed requires accurate data from river discharge or satellite images, which are rarely available prior to the 1980s. In the absence of these in situ measurements, proxies have successfully produced accurate, long-range, historical records of temperature, hydrological balance, and sediment discharge in coastal and oceanic environments. We present a 30-year, monthly resolved Ba/Ca proxy record of sediment in river discharge as measured from the skeletal remains of massive corals Porites sp. from northern Malaysian Borneo. We make the comparison with local instrumental hydrology data, river discharge and rainfall, to test the reliability of the Ba/Cacoral proxy. Our results show that averaging five records into two composites results in significant positive annual correlations with river discharge (r = 0.5 and r = 0.59) as well as a difference in correlation strength coherent with distance from the river mouth, with the composite closer to the river mouth displaying a higher correlation. More importantly, Ba/Cacoral records from this region showed a very similar upward trend to that of river discharge on multi-decennial timescales. The lack of similar increase and overall stability in the precipitation record suggests that the river discharge's trend recorded by corals is linked to the increasing land use associated with ever-growing deforestation. We argue that massive corals in this region are therefore valuable archives of past hydrological conditions and accurately reflect changes in land use patterns.

Coral geochemical response to uplift in the aftermath of the 2005 Nias–Simeulue earthquake

“…The variable response of Ba/Ca at the 1.8A fringing reef site also indicates that coral Ba/Ca is influenced by local variations (i.e., sediment delivery) and/or coral-colony specific factors. The results highlight the need for multiple coral Ba/Ca records for each reef location to provide composite records that resolve changes in sediment delivery 35 .…”

“…Delivery of sediments via runoff to the coastal zone is the primary source for these elements in nearshore regions, and coral proxy records have been widely used to reconstruct terrigenous runoff on seasonal and longer timescales in the tropics 29 . For instance, Ba/Ca in corals reflects changes in Ba in the surface-ocean reef environment related to a range of processes such as upwelling 31 , 32 , river discharge 33 – 35 , wind/wave associated sediment resuspension 36 and primary productivity and barite formation 37 , 38 . Mn/Ca in corals has been shown to track Mn cycling associated with large erosive events and delivery of suspended particulate matter through riverine discharge 39 – 41 .…”