Geochemical And Paleo-ecological Research On Ornithogenic Sediments From The Ross Sea Region

Global climate change is now laying substantial impact on the environment we are living in. To better understand the process and mechanism that how various drivers may influence the climate system of the earth, it is essential to study the historical change in the environment comprehensively, so that precise prediction for future climate change can be conducted. Adelie penguin is widely distributed around Antarctica, as one of the top predators in the Southern Ocean. The survival of Adelie penguin is heavily affected by many environmental factors, making the species an ideal target for research on the response of paleo-ecology to climate and environmental changes. The Ross Sea region has one of the biggest Adelie penguin concentrations nowadays with a long history of penguin occupation, and the ornithogenic sediments in the rookeries contain useful information of the past. In this study, we will analyze the samples with various techniques to interpret the influence that penguins imposed on the geochemical and isotopic composition of the sediments, and discuss the response of paleo-ecology of Adelie penguin to historical climate and environmental changes.1. Establishment of chronologies for the sediment profiles from the Ross Sea region210Pb-137Cs dating and AMS14C dating were employed to determine the chronologies of the ornithogenic sediment profiles from the Ross Sea region. Analysis for the activity of the radionuclides indicated that the zero point of210Pbex could always be reached in the profiles, and CRS mode was applied to calculate the age of the upper layers. Average flux of210Pbex in the profiles of this region is5.88Bq/m2a, similar to that of Antarctica region. Activity of137Cs in the sediments was generally low, and the maximum was often found in the surface, probably caused by additional137Cs brought by melt water from adjacent glaciers due to the warming climate in recent decades. According to the calculation of average deposition rate and combined analysis of210Pb-137Cs dating and AMS14C dating, the bottom layer of MB4, MB6, CL2, BI, CC, MB1, MR1and MR2were found dating back to394,1281,572,1656,1796,574,1631,1530AD, respectively.2. Identification of penguin bio-elements in the ornithogenic sediments and the eco-environmental implicationsSeabirds have substantial influence on geochemical circulation of elements, serving as a link for substance exchange between their foraging area and colonies. We investigated the elemental composition of eight penguin-affected sediment profiles including MB4, MB6, CL2, BI, CC, MB1, MR1and MR2. Among the three main constituents of the sediments (including weathered bedrock, guano and algae), guano was the main source of organic matter and nutrients, causing selective enrichment of several elements in each of the sediment profiles. In the23measured elements, As, Cd, Cu, F, P, S, Se and Zn were identified as penguin bio-elements in the Ross Sea region through statistical analysis and comparison with local end-member environmental media such as weathered bedrock, fresh guano and fresh algae. Compared with research in other parts of Antarctic, Arctic, and South China Sea, we found apparent overlap of avian bio-elements including As, Cd, Cu, P, Se, and Zn. Information on the composition and behavior of bio-elements in seabird guano on a global scale, and the role that bio-vectors play in the geochemical circulation between land and sea, will facilitate future research on avian ecology and palaeoclimatic reconstruction.3. Ecological succession recorded in sedimentary Hg and the preliminary pollution assessmentTotal mercury (Hg) concentrations were determined in profile MB4, MB6, CL2, BI, CC, MB1, MR1and MR2. Our data show significant positive correlations between Hg concentration and total organic carbon (TOC) content in all the profiles, suggesting the predominant role of organic matter (OM) as a Hg carrier. High Hg content in guano and a positive correlation between Hg and guano bio-element (phosphorus, P) in the ornithogenic sediment cores (MB6, BI, CC, MR1and MR2) indicate that Hg was strongly influenced by guano input. The bottom sediments of core MB6with seal hairs contain relatively high Hg. This increase is attributed to the input of seal excrement, suggesting that sedimentary Hg may be an effective trophic-level indicator from seals to penguins. The enrichment factors (EF) for Hg were calculated and the results indicated apparent Hg enrichment in the sediment cores from the Ross Sea region caused by bio-vectors such as penguins and seals. But sediment quality guidelines (SQGs) showed the total amount of Hg in the study area still rarely cause adverse effect to the local bio-communities.4. Distribution and sources of rare earth elements in ornithogenic sediments from the Ross Sea region, AntarcticaConcentrations of rare earth elements (REEs) were determined in profile MB4, MB6and BI. The distribution of REEs in each profile fluctuated with depth. REEs measured in environmental media and analysis on the correlations of∑REE lithological elements and∑REE-bio-elements in the profiles indicated that sedimentary REEs were mainly from weathered bedrock in this area, and the non-crustal bio-genetic REEs from guano and algae were minor. Further discussion on the slopes and Ce and Eu anomalies of chondrite-normalized REE patterns indicated that a mixing process of weathered bedrock, guano and algae was the main controlling factor for the fluctuation of REEs with depth in the sediments. An end-member equation was developed to calculate the proportion of REEs from the three constituents in the sediments. The calculation functioned well in estimating bedrock-derived REEs and the magnitude of ornithogenic influence in different profiles. In general, REEs in the ornithogenic sediments showed anti bio-element patterns and thus can be used as an additional proxy to reconstruct historical penguin populations.5. Environmental implication of carbon and nitrogen isotopic composition in ornithogenic sedimentsWe analyzed δ13C, and δ15N in both acid-treated and untreated sediments from profile MB4, MB6, CL2, BI and CC that were influenced by penguin guano. Carbon isotopic composition in the ornithogenic sediments showed a mixing feature of guano and algae. Using a2-member isotope mixing equation, we were able to reconstruct the historical change of guano input and algal bio-mass. The difference between treated and untreated δ15N (△15N) was significant in three profiles which were heavily impacted by guano, and minor in two profiles with less guano influence. We determined that total nitrogen in the sediments is primarily derived from penguin guano and algae, and used an N-species test to explain the variation of△15N in two profiles. It was found that post-depositional decomposition and ammonia volatilization, which have important roles in the cold and arid environment of Antarctica, would render an elevated δ15N through kinetic isotopic fractionation in the inorganic nitrogen from guano. N-species analysis revealed that the percentage of inorganic nitrogen in total nitrogen, indicative of the degree of guano influence, is the key factor controlling△15N in the sediments. This hypothesis successfully explained nitrogen isotopic composition in the remaining three sediment profiles. We conclude that the parameter A15N, rather than traditionally used untreated δ15N, can be taken as an effective proxy for the strength of avian influence on ornithogenic sediments in the Ross Sea region.6. Paleo-ecology of penguins and seals in the Ross Sea region and the comparison with ice-free areas around AntarcticaDue to the sensitivity to environmental changes, penguins in Antarctica are widely used as bio-indicators in paleoclimate research. On the basis of bio-element assemblages identified in ornithogenic sediments, we reconstructed the historical penguin population change at Cape Bird, Ross Island, for the past1600years. Clear succession of penguin population peaks were observed in different profiles at about1400AD, which suggested a high probability of internal migration within this region. The succession was most obviously marked by a sand layer lasting from1400to1900AD in profile MB4. Multiple physical/chemical analysis indicated this layer was not formed in a lacustrine environment, but was marine-derived. We believe that colder condition during the little ice age (LIA) would lead to increased ice mass on Ross Island, causing isostatic subsidence and inland-ward movement of the shoreline, which in turn resulted in the abandonment of the colonies. The penguins migrated from the coastal area of mid Cape Bird northward and to higher ground as documented in other sediment profiles, indicating an ecological response to global climate change and subsequent geological effects in Antarctica. Several shorter profiles from Cape Crozier, Cape Royds and Beaufort Island showed that the penguin population only stated to rise in the most recent200years due to a warming climate, corresponding to various geological and instrumental records.7. Preliminary research on seal ecology at Marble Point The seal hairs excavated from Marble Point, Victoria Land recorded the presence of seals at around2700a BP, in accordance with the "seal optimum" with warmer climate and open water.