四海龙湾玛珥湖人类世地层颜色变化原因及其指示意义

Mechanism of Anthropocene sediment colour change in Sihailongwan Maar Lake and its significances

关键词:  冷冻岩芯  溶解氧  氧化还原电位  人类世  沉积物  色度

Key words:  freeze core  DO  ORP  Anthropocene  sediment  chromaticity

摘要:

湖泊沉积物颜色变化往往能够反映古气候和环境的演变。在中国东北的四海龙湾玛珥湖采集沉积物冷冻岩芯，发现表层20 cm岩芯的颜色随深度具有明显变化：从顶部的灰黑色向下变为灰黄色再到底部的棕黄色，并且在5—7 cm深度发育一条白色标志层。基于采集的冷冻岩芯，分析了过去200年以来沉积物 （对应于表层 20 cm 的岩芯） 的颜色变化特征，重建了元素含量、溶解氧 （DO） 和氧化还原电位 （ORP）的历史变化，并结合有机质、磁化率等指标，对沉积物颜色变化机理及指示意义进行了探讨。结果表明：1977—1978年的白色标志层可能是龙湾地区大规模森林砍伐后短时间内大量外源碎屑物质输入形成的，可作为龙湾地区玛珥湖沉积物定年的一个参考时标；沉积物颜色的快速变化发生在20世纪中叶，其直接原因可能是沉积物ORP下降、金属硫化物增加导致的，根本原因可能是20世纪的全球升温导致湖泊DO下降的结果。四海龙湾玛珥湖沉积物颜色变化反映了20世纪中叶湖泊水体和沉积物还原性增强的特点，进一步反映了大加速时期沉积环境的改变。沉积物颜色可作为识别人类世地层的一个重要指标，支持人类世开始于20世纪中叶。

Abstract:

Background, aim, and scope The colour change of lake sediment often reflects the change of the paleoenvironment and paleoclimate. Sihailongwan Maar Lake (SHLW) is a key spot in the study of Anthropocene stratigraphy in China. Distinct colour changes were observed in the Anthropocene sediments: with dark colours in the upper of the core, transitioning to dark-yellow bands and then to brown-yellow in the lower parts. In addition, there is a distinct white-coloured marker layer in 5—7 cm in the cores. Our aim is to investigate the factors of sediment colour changes and the formation mechanism of the marker layer, and further to discuss the significance of sediment colour as an indicator of the Anthropocene. In this study, the freeze core of SHLW over the last 200 years was used as archives. The characteristics and influencing factors of changes in Anthropocene sediments were analysed. the physicochemical characteristics of SHLW were investigated. Materials and methods We collected freeze cores in SHLW (42°17′4.48″N, 126° 36′4.48″E) in February 2021. For the top 20 cm of core SHLW21-Fr-16A, accurate chronologies of the last 200 years were established using varve dating. Based on high-definition photos of the freeze core (SHLW21-Fr-16A) taken by a single lens reflex camera at −20 ℃, image processing software was used to obtain colour parameters in RGB and CIELAB modes. The dissolved oxygen (DO) and oxidation-reduction potential (ORP) of the parallel core SHLW21-Fr-14A were measured using a portable Redox potentiometer. The element abundance of the core SHLW21-Fr-14A was measured using micro-X-ray fluorescence (XRF). Results The colour changes in the sediment primarily occurred in the mid-20th century. The DO concentrations of the entire core were lower than the detection limit (0.01 mg/L). The ORP of the cores were all negative and declined rapidly around 1950, and the average ORP before and after 1950 were −54 mV and −143 mV, respectively. Significant changes occurred in the abundance of the following elements: Iron and sulphur began to increase slowly around 1950, which became rapid around 1970s and 1980s, respectively. Discussion The age of the white-coloured marker layer was previously determined to be 1977— 1978. In the marker layer, rock-forming elements, Rb/Sr and Ti/Ca values, and magnetic susceptibility have peak values, indicating the increase of exogenous detrital or dust material input. Further investigation in the local area found that a large-scale deforestation event occurred at Longwan Forest Farm in 1977—1978; therefore, we speculated that the white marker layer may have been formed from the increase of local dust caused by deforestation events. In terms of the colour or chromaticity changes in SHLW sediments, they changed significantly in the mid-20th century. In reducing environments, sulphur (S) and iron (Fe) elements exist in the form of S2− and Fe2+ , respectively. Our result shows that the abundance of S and Fe elements increased since mid-20th century in SHLW sediments. The increase in S may result from increased fossil fuel emissions since the Great Acceleration, whereas the increase in Fe may result from increased input of detrital material. The ORP results show that the reducibility of sediments increased in the mid-20th century, which is conducive to the formation of FeS. Consequently, the increase in FeS levels in the mid-20th century may be the main reason for the darker sediment colour. In addition, in SHLW sediments with increasing reducing properties, FeS will not be oxidized, allowing colour characteristics to be preserved for a long time. Therefore, the sediment colour can serve as an important physical indicator of the Anthropocene in SHLW. The fast decrease of ORP in the mid-20th century reflects the change of sedimentary environment in SHLW. It may be the result of the decrease of DO concentrations in the deep water caused by global warming, as reflected in the records of many temperate lakes worldwide. However, owing to the lack of multi-year observation records of DO concentration in SHLW, more evidence is needed to prove it in the future. Conclusions (1) The age of the white-coloured marker layer in the Anthropocene strata is 1977—1978, and it was caused by the increase of dust imported into the lake due to large-scale deforestation in the Longwan area at that time. It can be used as an individual time marker of the Anthropocene stratigraphy in maar lakes in the Longgang Volcanic field. (2) The increase of sulphides in the Anthropocene strata is responsible for the change of colour from yellow to dark, which is the result of the decrease of DO content in the bottom lake water and the enhanced sediment reducibility since the mid-20th century. (3) The change in sediment colour in SHLW reflects changes in the climate environment since the mid-20th century, supporting the onset of the Anthropocene in the 1950s. Recommendations and perspectives To further validate the findings of this study, future work should involve measuring FeS content using chemical methods, investigating DO in the water column, and extending related research to other maar lakes within the Longgang volcanic group.