Comparative Study On Eco-environmental Records In Typical Antarctic And Arctic Areas Over The Past3000Years

Global change research is the subject of global concern. Since the implementation of the In-ternational Geosphere-Biosphere Programme (IGBP), the research on global change has become the forefront of Earth Sciences. The more limited interval of the past two millennia, a period that can be referred to "late Holocene", is of particular significance. The principal boundary conditions on the climate (e.g., planetary orbital geometry and global ice mass) have not changed appreciably since then. The variations in climate observed over this time frame, therefore, are likely repre-sentative of the natural climate variability that can be expected in the present century if without any human influence. Investigating modern climate change, including global warming, in a long-er-term context thus can help establish the magnitude of anthropogenic forcing on the recent past and future changes. On basis of modern observation data accumulation, polar eco-environmental research emphasizes on long time scale climate reconstruction, to study the climate evolution mechanism and its impact on the eco-environment. Climate change has great impact on freshwater ecosystems. But how can the fragile polar lake ecosystem respond to the warming climate? The spatial and temporal changes in the poles are poorly documented and understood, because long-term monitoring data are scarce. In addition, whether the eco-environment is changing and how to differentiate the impact of climate change and human activities, are also key questions in order to predict the future changes in lake ecosystems. In this paper, we focus on the late Holocene which is closely associated with human activities, and on lake and peatland sediments from Ant-arctica and Arctic, respectively. A number of sediment sequences as well as environmental media including lake surface samples, algae and surrounding soils were collected, to probe the natural and anthropogenic factors in the lake ecological evolution. Main contents and results are given as follows:1. History of ecosystem responses to environmental variations from a lake in ArcticBy employing the multi-proxy analyses (including sediment pigments, mineral magnetic susceptibility, diatom composition, carbon and nitrogen isotopes), combined with other physical and chemical proxies such as calcium carbonate, total organic carbon, we have reconstructed the historical changes of lacustrine primary productivity in Ny-Alesund over past3000years. Our data showed that during the period of Little Ice Age (LIA), the climate condition was unfavorable for the growth of lake algae, and thus the lacustrine productivity declined. This result was sup-ported by the relatively low contents of pigment and biogenic silica in the sediments. Correspond- ing to the warming climate after LIA, the contents of total organic carbon (TOC) and sediment pigments increased significantly in the upper5cm (～1890AD), which reflects the rapid growth of lake algae and thus the great increase of lacustrine primary productivity. However, the biogenic silica in the upper sediments remained relatively low, which might be related to the growth com-petition with other algae species. Over the past100years, the ratio of Osc/Myx in the sediments decreased continuously, indicative of durative increase of myxoxanthophyll in blue green algal pigments, and this might imply that the human activity had enhanced the nutrition level of the lakes in the Arctic region. Based on the comprehensive study from element ratios, mineral compo-sition and chemical altered index (CIA), we found that the Ny-Alesund region had a relatively warm climate and oxic condition prior to550cal. yr BP, and that the lake basin experienced stronger chemical weathering at this time. During the period～550-100cal. yr BP, the intensity of chemical weathering around the lake catchment had weakened, and the algae biomass produced in the lake declined, corresponding to the increased time of lake ice cover and closed sedimentary environment during LIA. At the beginning of the20th century, the chemical weathering and sur-face runoff remarkably increased, which is consistent with the rapidly warming climate and plenty of the precipitation during the last100years. The present study reveals that Ny-Alesund has a similar palaeoenvironmental record to Svalbard and the wider circum-Arctic region, suggesting that the environmental change in Ny-Alesund is closely linked to larger scale arctic change. The study also shows that the lake primary productivity has obvious response to climate and environ-mental change in arctic.2. Heavy metal records in Ny-Alesund, SvalbardUsing lake and peat sediments, we discussed the distribution of heavy metals in past3000years, and employed the four kinds of evaluation index to preliminarily assess the heavy metal pollution in Ny-Alesund. The results showed that the pollution degrees of As, Hg and Pb were relatively high, which can represented as "mild" or "moderate" pollutions, while the other heavy metals belong to "no or very low pollution" degrees. The three distinct peaks of Hg deposition flux recorded in past200years were corresponding to1800s,1880s and1970s. By comparing the historical Hg deposition flux with the records of local coal combustion and globe Hg production, we suggest that anthropogenic Hg is mainly derived from the long-range atmosphere Hg transport, whereas the influence of local coal mining is likely minor. Ny-Alesund has similar Hg pollution degree with Sweden. Canada and other circum-arctic areas, but its pollution degree is obviously higher than Greenland and significantly lower than America as well as other industrialized coun-tries. Quantitative calculation on the biogeochemical proxies (e.g., pigments and diatom biomass) and Rock-Eval analyses showed that89.6-95.8%of the Hg in post-1950could be explained by scavenging. The distribution of MeHg has a close relationship with total Hg and organic matter. The redox condition is one of the possible factors affecting the methylation rates in lake sediments. Higher algal productivity and organic matter actually led to the increase of methylation in the up-permost sediment. The anthropogenic Pb in Ny-Alesund largely originated from W. European and Russia, and the local coal mining is not the main Pb contributor. The peat profile clearly reflected the historical changes of atmospheric deposition of anthropogenic Pb into Ny-Alesund. The result showed that anthropogenic Pb peaked at1960-1970s, and a significant recovery was observed thereafter by a rapid increase of206Pb/207Pb ratios and a remarkable decrease in anthropogenic Pb contents. In contrast, the recovery signal of Pb pollution in the last decades is not present in the surface lacustrine sediments, and this might result from the increased occurrence of the cli-mate-sensitive processes (e.g., surface erosion; precipitation and meltwater runoff, etc.) caused by the recent rapid warming in Arctic.3. Lake eco-environment in response to climate changes in AntarcticaWe collected two lake sediment cores from the East Antarctic region and analyzed biogenic silica and other biogeochemical parameters (e.g., organic matter, C, N, S, H). Based on syntheti-cally comparative research, we found that the contents of biogenic silica in the lacustrine sedi-ments can sensitively indicate the evolutionary history of paleo-primary productivity, and thus BSi is an ideal proxy for reconstructing eco-environmental change recorded in the lacustrine sediments in East Antarctica. Multi-proxies including C, N, S and H in the kerogen, TOC, biogenic silica, total amino acid, Rock-Eval parameters in the bulk MC sediments were used to reconstruct the changes in lake paleoproductivity, and some geochemical parameters such as δD, C/S and mag-netic susceptibility (χlf) revealed the historical climatic fluctuation in the study area during the past3000years. According to the paleoproductivity and paleoclimate reconstructions, we found that the historical primary productivity recorded in the Mochou Lake displayed distinct changes over the past3000years, with four low stages corresponding to～2500-2200,1900-1650,1400-1200, and900-300cal. yr BP. Comparison of paleoclimatic data from some other regions in East Antarc-tica showed that our study area experienced similar climate conditions with those areas historically, such as Neoglacial period and Little Ice Age, although their onset and duration times might vary in the different regions. Our comparative study suggested that the Antarctic lake ecosystem has been closely linking with climate change, where cold climate is unfavorable for the growth of algae and therefore reduces the lake primary productivity.4. Heavy metal records and comparative study in circum-Antarctic areasEight sediment profiles (including lake, peat and ornithogenic sediments) were collected from circum-Antarctic areas (including Ardley Island and Byers Island in west Antarctic, Ross Sea regions and Larsemann Hills in East Antarctica). We compared the heavy metal contents, and em-ployed the three index (i.e., enrichment factors-EF, Geoaccumulation index-Igeo and Modified de-gree of contamination-mCd) to assess the pollution degree in each profile. By comparison with the concentrations of heavy metals in environment medium (e.g., soil, rock, dropping and moss), we discussed the pollution factors including bedrock background, seabird dung input and atmospheric deposition from human activities. We also used the sediment quality standard (SQG) to evaluate the ecological effect of eight metal elements (Ni, Zn, Cu, Cd, As, Pb, Hg and Cr). The results show that the heavy metal pollution is more severe in ornithogenic sediments. The contents of Zn, Hg, Pb and As were low and no pollution effect happened, while other metals such as Ni, Cu, Cd and Cr reached the level of ecological risk in some sediments, which are related to the guano input and local geologic background.5. Comparative study on the ecological record and environment evolution in late Holocene in Antarctic and Arctic areaThe climate trend in High Arctic was similar to that in China and other mid-latitude regions, namely experienced "warm-cold-warm" period in the late Holocene, during which the "medieval warming period" and "Little Ice Age" as well as the recent warming after20th century are the most evident. Although the reconstruction of time and magnitude of climate change were not all consistent because different latitudes and proxies were chosen, they all reflect the overall similar trend in Northern Hemisphere. The climate change trend in Antarctic has some differences from that in Arctic, and the changes such as the amplification effect and duration time has significant regional variations. Our reconstruction from lake sediments in Antarctica and Arctic suggest that the lake eco-environment (e.g., lake primary productivity, redox condition) and the catchment ge-ochemical environment (e.g., weathering and hydrodynamic processes) have active response to climate change. The results of heavy metal pollution in Polar Regions show that different materi-als enrich different levels of heavy metals, and pollution of the lake and peat sediment were lower than that of ornithogenic sediments. The heavy metal pollution in Antarctic was significantly higher than that in Arctic, which was caused mainly by the guano input.