Jellyfish Lake, Palau: Early Diagenesis of Organic Matter in Sediments of an Anoxic Marine Lake

Jellyfish Lake, a meromictic, marine lake in the Palau Islands, western Pacific Ocean, is accumulating organic matter-rich sediments (46% wt/wt) below anoxic, highly sulfidic bottom water. Elemental and stable isotope $(\delta^13C)$ data indicate that sedimentary organic matter is primarily derived from vascular plant remains (65-90%) and would form a type 2 kerogen. A bacterial layer at the oxic-anoxic interface in the water column may play an important role in the rapid recycling of algal-derived organic matter from the highly productive surface waters. The major postdepositional change in the sedimentary organic matter is carbohydrate biodegradation, as shown by $^13C$ NMR. Lignin and aliphatic substances are preserved in the sediments. Dissolved organic matter in pore waters is primarily composed of carbohydrates, reflecting the degradation of sedimentary carbohydrates. Rate constants for organic carbon degradation and sulfate reduction in sediments of the lake are about $10\times$ lower than in other anoxic sediments. This may reflect the vascular plant source and partly degraded nature of the organic matter reaching the sediments of the lake. Elemental results from sediment cores and stable isotope $(\delta^18O, \delta^13C)$ studies of shells from the sediments indicate probable climatic or ecological fluctuations affecting the depositional environment of the lake between 80 and 110 yr ago. The high sedimentation rate and stable environment of Jellyfish Lake make it a possible environment for studies of climatic fluctuations during the Holocene.