Did eustatic sea-level control deep-water systems at Milankovitch and timescales?: An answer from Quaternary Pearl River margin

Abstract

As one of the most important forcing factors, sea level fluctuations exert a major influence on deep-water depositional processes, however, it is still not well understood how they control the evolution of the specific deep-water system over different timescales. For relatively longer (1 My) and shorter (100 Ky) timescales, we characterize deep-water sedimentary records on the Pearl River margin using seismic and borehole data, and then compare them with the contemporaneous sea-level curve to exam the varied roles sea levels have played in impacting the development of the deep-marine system. Results indicate that over both the 1 My-scale and the 100 Ky-scale, the studied deep-water system shows systematic variations that are strongly suggested to arise from the modulation of sea levels. Specifically, over the 1 My-scale, facies and depocenters of the deep-water system all show significant changes. Large fan lobes (consisting of both turbidites and mass-transport deposits) with more distal depocenters likely correspond with low-amplitude and high-frequency sea level fluctuations and slightly rising shelf edge trajectories, whereas smaller turbidite fan lobes with more proximal depocenters likely reflect high-amplitude and low-frequency sea level fluctuations and steeply rising shelf-edge trajectories. In contrast, within the 100 Ky-scale of a glacial-interglacial cycle, the composition of deep-water deposits also shows significant variations. Coarser-grained deposits with higher organic carbon (TOC) content and lower calcium carbonate (CaCO3) content are interpreted to reflect periods of glacial sea-level lowstand, whereas finer-grained sediments having lower TOC and higher CaCO3 content reflect interglacial sea-level highstand. Moreover, the spectrum characteristics of these constituent curves are very similar to those of contemporaneous sea levels, reflecting Milankovitch climatic forcings and further validating the tight coupling between sea-level stands and sediment compositions. These correlations between sedimentary records and sea level behaviors suggest that it was mainly a long-term change in the amplitude and frequency of eustatic cycles that controlled the overall architecture of the deep-water system, whereas it was shorter term, changing sea-level stands that played a role and impacted the deep-water sediment composition.