渤海湾西岸全新世海面变化

在讨论海面标志点基本定义的基础上,从渤海湾西岸534个原始数据中获得了136个指示高水位、潮间带或低水位的海面变化标志点,并重建了它们的相对海平面时空分布,进而获得了渤海湾西岸全新世相对海平面变化带.约10cal.ka B.P. 时,相对海平面变化带达到-25m,以约6m/1000a(即约6mm/a)的平均速率上升,在6cal.ka B.P. 前后达到现代海面的高度; 6cal.ka B.P. 至今,变化带的高度介于+1m~-2m之间,未发现中全新世相对高海面.再搬运海相贝类和陆相泥炭类样品的 14C 年龄,分别存在约600年和660年的驻留时间.经过驻留时间校正的新海面变化年代学序列,将渤海湾相对海平面达到现代高度的时间点确定为约6cal.ka B.P.,从而与全球海面变化的对比更为准确.渤海湾盆地的长期稳定下沉和沉积自重压实的共同作用,可能抵消了冰川均衡调整(GIA)引起的中全新世数米高的相对高海面.

HOLOCENE SEA LEVEL CHANGE ON THE WEST COAST OF THE BOHAI BAY

The west coast of the Bohai Bay, as one of the pioneer areas, was selected for Holocene sea level change study in China since the mid-1970s. Not only common elements such as the basal peat layers but also both Holocene cheniers and oyster reefs, localizing particularly in the area, were continuously used as sea level indicators for local sea level reconstruction. Based on fundamental studies, given by the others during the last four decades, and the recent finds by the autors group of this paper, whose specific efforts were mainly focused on the development of cheniers and oyster reefs, and Holocene coastal sedimento-and chronostratigraphies, this paper thoroughly refined indicative meanings of the local sea level indicators. Consequently, 136 Holocene sea level indicators were obtained from 534 original data, including 533 radiocarbon-dated samples and an archaeological date, from depositional sediments, cheniers and oyster reefs. These indicators, consisting of three categories: being formed at high waters, within intertidal zone and at low waters, respectively, were then carefully converted to their mean sea level meanings.Reconstruction of a relative mean sea level band, derived from the spatial and temporal distribution of such indicators, revealed that the local mean sea level reached to -25m at around 10cal.ka B.P. Following sea level rise with average rate of 6m/1000a(i.e., 6mm/a), this relative mean sea level band approximated to its present level at around 6cal.ka B.P., and then the band remained at +1~-2m till the present. Although GIA models preliminarily predicted existence of the Mid-Holocene highstands of +2.0~+2.4m, there are no any obvious evidence actually showing such highstands in the working area based on our observed sea level indicators. Coexistence of the long term subsiding background in the study area and surrounding region and possible self-consolidation of sediments is a plausible reason to offset the GIA-induced Mid-Holocene highstand.Both reworked shells and peaty layers are common and indispensible dating elements for Holocene muddy coast chronology. However, dates given by such reworked materials are not able to reveal proper depositional ages because of the residenece time effect. Based on systemaic AMS 14C dating test at the subsample-level under a single layer or a bulk sample, we provided two sets of semi-empirical values of (1)600cal a, and (2)660cal a for the local residence time correction of both reworked marine shells and terrestrial peaty samples, respectively. Thus, a new residence-time-calibrated choronology after common CALIB procedures indicated that the local sea level indicators and the mean sea level band actually reached to their present altitude at around 6cal.ka B.P. This amended timing of sea level history provided a better understanding of intrinsic relationship to the non-residence-time affected global eustatic sea level trend, and will have potential benefit of better fit of GIA model predictions to the observed signals.