Refined chronostratigraphy of a late Quaternary Sedimentary sequence from the Yangtze River delta based on K-feldspar luminescence dating

摘要

Dating of sediments in the tidal-dominated Yangtze River delta remains a challenge because of extremely complicated processes during sediment transport and deposition in such a fluvial-marine transition zone. To better understand the late Quaternary sedimentary history of the Yangtze River delta, we explore the applicability of K-feldspar luminescence dating techniques to deposits through a case study on a 60-m-long borehole core (YZ07) in the delta and then establish a spanning similar to 50,000-year-long chronological sequence. A series of luminescence pre-tests, including dose recovery tests and determination of residual dose and fading rates, were implemented using the modified single-aliquot-regenerative dose (SAR) protocol for post-infrared (IR) IR stimulated luminescence (IRSL) measured at 150 degrees C after prior IRSL at 50 degrees C (pIRIR(150)), respectively. Luminescence characteristics of the prior IRSL (IR50) and pIRIR(150) were then analyzed to check their validity. Two fading-correction models were utilized to correct apparent ages obtained from IR50 and pIRIR(150) signals. The newly obtained K-feldspar luminescence ages were compared with quartz OSL ages and C-14 ages. Our results indicate that most of ages derived from pIRIR(150) signals were overestimated due to incomplete bleaching; in contrast, the fading-corrected ages from IR50 measured in pIRIR(150) protocol are robust as they are consistent with quartz OSL ages and C-14 ages as well as the stratigraphy. Additionally, IR50 ages are characterized by negligible residual dose, and thus used for defining chronology of the investigated sequence over the past 50 ka. In combination with sedimentological and paleontological analyses of core YZ07, we find that fluvial to fide-influenced fluvial deposits were emplaced during the period 30-50 ka, which were then overlain by post-glacial tidal-river (13-11.5 ka), and Holocene estuarine-shelf, delta system, and tidal flat deposits in the Yangtze River delta. Together with other chronostratigraphy from the selected cores, we confirm the existence of Marine Isotope Stage (MIS) 3 depositional units along the marginal coast of the Yangtze River delta in relation to the interstadial sea-level highstand. Our stratigraphic correlation based on the refined chronology reveals offshore transition of the MIS 3 unit into facies deposited in deeper and offshore marine environments.