Analyzing the Climatic History of Sisimiut Using Isotopes - Coursework Example

Analyzing the climatic history of Sisimiut using isotopes al Affiliation) Contents Contents 2 0 3 2.0 INTRODUCTION 3 3.0 DISCUSSION 4 3.1 Oxygen and hydrogen isotopes in lake waters 5 3.2 Carbon isotopes in the lake waters 8 3.3 Analysis 8 4.0 CONCLUSIONS 10 5.0 REFERENCES 12 1.0 ABSTRACT An analysis of climatic and paleoceanographic changes in Sisimiut, in Southwest Greenland, indicates that the region is sensitive to differences in the interaction between the two components of the West Greenland Current; the Polar Water from East Greenland Current and the Atlantic Water of the Irminger Current. Oceanographic changes, mid- and late Holocene, are indicated by a seismic profile of the development in the region since the deglaciation (Leng, & Anderson, 2014). These changes are also illustrated by an analysis of stable isotopes (δ13C and δ18O), benthic foraminifera, and X-ray fluorescence counts. The chronology is determined by AMS 14C and 210Pb. In the interval of 6650-3800 cal, yr BP, a high Si/Fe ratio and depleted δ18O values indicate warm conditions in relation to the Holocene Thermal Maximum (Zimmerman, 2001). These conditions are followed by a slight cooling that fluctuates after 3800 cal. Yr Bp. The δ18O and foraminifera assemblage indicate cold and low-salinity bottom water conditions. These conditions correspond to a minimum solar irradiance and the Dark Ages Cold Period; that is, a Medieval Warm Period (MWP) (Erbs-Hansen, Knudsen, Lykke-Andersen, Underbjerg, Sha,2013). 2.0 INTRODUCTION Lake waters in Sisimiut have a stable-isotope composition that has been measured (temporally and spatially) and compared to the local climatic information (Battarbee, & Binney, 2008). A climatic gradient exists whereby; the coast is characterized by a maritime climate, while the mainland has a low Arctic continental climate. The maritime climate indicates minimum yearly temperature range and increased precipitation. The climate change from maritime to continental is indicated in the isotopic composition of the lakes (Mackay, 2005). The inland lakes have values δD and δ18O that lie on evaporation trends far from meteoric water composition. The coastal lakes have δD values and δ18O that lie close to the meteoric water composition. Long-term variations superimpose short-term seasonal trends. For example, isotopic enrichment in smaller lakes has been experienced in the relatively dry years, which buffer the larger lakes in their response to short-term changes (Erbs-Hansen, Knudsen, Lykke-Andersen, Underbjerg, Sha,2013). The inland lakes have δ13C contributions from the exchange with isotope-rich atmospheric carbon dioxide and organic carbon. δ13C has been depleted in the small coastal lakes due to organic materials. This study indicates the importance incorporating local hydrogen settings to interpret the lacustrine isotope records. In Sisimiut, this information enhances the identification of lakes that are capable of recording arid intervals, because evaporation/precipitation adjustments towards greater aridity may not be indicated in the Greenland ice-core records. 3.0 DISCUSSION The isotopic composition of oxygen in biogenic carbonates is a suitable paleoclimate proxy. Through-flow lakes provide a platform for the study of past precipitation patterns, since waters in these lakes reflect the isotopic composition of rainfall. Diatoms do not exhibit changes in their isotopic compositions. This is caused by post-depositional diagenesis, where their isotopic composition is maintained after burial. Lake hydrology (the inflow to outflow ratio) and precipitation influence the isotopic composition of lake water (Ghosh, & Brand, 2003). Mean time residence should be timely enough; that is, it should be long enough to capture precipitation patterns and short enough to reduce fractionation caused by evaporation. If the mean time residence time is accurate, the isotopic composition of lake water will sufficiently establish the yearly mean isotopic composition of precipitation. The presence of numerous through-flow lakes in Sisimiut, such as; Sisi 15 and Tasersuaq, present an important opportunity to utilize oxygen and carbon isotopes. This understanding is vital for the analysis of local and regional precipitation patterns and atmospheric circulation during the Holocene. 3.1 Oxygen and hydrogen isotopes in lake waters Stable isotopes are conservative in nature. Local climate; in most cases, temperature, affect the average annual values for δD and δ18O (Dawson, 2007). These values are altered by physical processes (evaporation and mixing). The chemical and isotopic compositions of the lake water are altered by evaporation and mixing of waters from various reservoirs. Water isotopes are distributed during this process providing a distinctive signature. The correlation between δD and δ18O is referred to as the Global Meteoric Water Line (GMWL). This correlation is indicated in the formulae; δD=8 δ18O+ 10‰. This relationship can be indicated using the δD versus δ18O diagram below (Seidov, Haupt, & Maslin, 2001); Figure 1, Sisimiut stable isotope data diagram. The slope of the Local Evaporation Lines (LEL) is derived from the ratio of stable isotope enrichments for D and 18O. This ratio depends on local climatic factors such as; seasonality of precipitation, origin of the vapor mass, and evaporation during and after precipitation. The slope, being less than 8, indicates that lake waters appear to be similar to meteoric water. This may not be the case as some may be evaporated relative to their initial compositions while others may have a higher amount of summer rainfall due to variable lake water residence times (Volkman, 2006). The slope of the isotope data represents a function of the local climate gradient in Sisimiut, which influences both the rainfall and the amount of evaporation from the lakes. These effects are complex but are unraveled by the strong regional signals in the whole data set where the climatic gradient plays some role. Lower humidity experienced close to the ice sheet is indicated in the isotope data diagram. δD and δ18O in lake waters in Sisimiut have been compared to the Global-MWL. The lake waters either exhibit similarities to meteoric in composition or lie on trend lines away from the Global-MWL. Evaporation is the main cause of enrichment of δD and δ18O and preferential loss of the lighter isotopes. From the diagram above, it is evident that a well defined local evaporation line can be preserved under ice cover. This LEL captures the signal of evaporative isotope enrichment achieved during the pre-melt water season (Kemp, Vane, Horton, Culver, 2010). Maritime climate at the coast experiences lower rates of evaporation, which ensure that lake waters are the same as meteoric water. Low precipitation and high rates of evaporation lead to enrichment in both δD and δ18O in inland lakes. Intersections of the Global-MWL with all local evaporation lines are approximately δ18O=-19 ‰. This can be used to estimate the average weighted yearly precipitation for Sisimiut. This value is consistent with the intermediate geographical location of Sisimiut from Gronnedal to the south and Thule in the north which have average weighted values for annual precipitation of -12‰ and -22‰ respectively (Dawson, 2007). 3.2 Carbon isotopes in the lake waters Not all the lakes in Sisimiut have sufficient dissolved inorganic carbonate. Some lakes are too dilute to provide carbon isotopes. δ13C values range from -25 to 0‰. The coastal lakes have the least δ13C values compared to inland lakes. In general, ground waters (usually in the form of HCO3-) have the least carbon isotope composition due to the presence of light carbon isotopes from organic matter. Lake waters with low δ13C tend to have low δ18O. The exchange of atmospheric CO2 on inland lakes affects the δ13C compositions. These lakes have longer residence times or volume ratios. The extent of photosynthetic removal of 12C also affects the carbon-isotopic composition in the lake waters (Dawson, 2007). 3.3 Analysis Lake sediment isotopic composition is used as paleoclimate indicators. The isotopic composition of carbon and oxygen (in lacustrine biogenic silica) has proven to be a valuable proxy in the quantitative estimation of changes in precipitation patterns, temperature, and evaporation that influence climate in global ecosystems. The climatic gradient in Sisimiut is reflected in the isotopic composition of the lake waters. The inland lakes have δD and δ18O compositions that lie on evaporative trends away from the meteoric water, whereas the coastal lakes have δD and δ18O compositions close to the MWL. Seasonal changes in the isotopic composition are superimposed on the geographical controls. For the smaller lakes, the isotopic composition is ‘reset’ by input of snowmelt water with light isotopes (Leng, 2006). Snowmelt enhances evaporation in the summer period and readjusts the isotopic composition in the lake water (Gerhard, Harrison, & Hanson, 2001). The level of isotopic change of lake water is a function of the amount of snowmelt, the original isotopic composition, and the evaporative loss in the subsequent period. Carbon isotope ratios in the lake waters are interpreted in terms of various sources of carbon. The smaller lakes have light carbon isotopes while large inland lakes have heavy carbon isotopes from their interaction with heavier atmospheric CO2. The amount of isotope equilibration with atmospheric CO2 is a function of lake surface area (Taniguchi, & Holman, 2010). Inland lakes fall along the Global-MWL, confirming reduced isotopic effects of evaporation. Sediments have high concentrations of diatom compositions dominated by a single genus. These lakes are ideal sites to indicate previous atmospheric circulation patterns in a region with insufficient data (Aggarwal, Froehlich, & Gat, 2005). The water-isotope data characterizes the regions hydrological settings that influence the isotopic composition of precipitation in lacustrine sediments. Adverse climate changes such as; temperature, the ratio of inflow to evaporative fluxes, and relative humidity alter the isotopic composition in lake waters by more than 1.0 ‰ individually. In natural systems, these variables do not change independently. They have a causal and effect relationship; for example, cool temperatures reduce the rate of evaporation. A change in the isotopic composition of rainfall has a huge effect on the change in isotopic composition of lake waters. Large fluctuations in isotope (above 1.5‰) reveal changes in precipitation patterns. Smaller fluctuations are difficult to interpret as they indicate a combination of hydrological and climatic factors (Kemp, Vane, Horton, Culver, 2010). Ice cores in the region provide evidence for Late Holocene changes in rainfall patterns. Increase in terrestrial dust and sea salt indicate intensified meridional air flow. Diatom inferred conductivity reflect decadal to centennial changes in effective precipitation patterns. This indicates a change in the location of the Baffin Trough with minimal change of the Late Holocene. The mid-Holocene is a region of severe climatic change. An examination of the deeper sediment cores indicates major climatic fluctuations in the early to mid-Holocene. Multi-proxy analyses and Holocene thermal maximum indicate the likelihood of precipitation patterns and temperature suggested by changes in the oxygen and carbon isotopes (Leng, & Anderson, 2003). 4.0 CONCLUSIONS The isotopic composition of lake waters is directly influenced by the amount of precipitation. Smaller lakes are more responsive to this condition. Larger lakes have longer residence time. This enables them to capture long-term, progressive climatic change; promoting isotopic equilibrium with the atmosphere. Lakes in southern and western Greenland respond to local climatic changes. Modern relationships indicate that climatic changes in the past have been recorded in diatom silica and lake water isotopic compositions. Short mean residence times influence lake water to respond to seasonal climate. This means that isotopes in lake water are controlled by the air temperature (Könitzer, Leng, Davies, Stephenson, 2012). Oxygen isotopes in lake water and oxygen and carbon isotopes in carbonates have been the main paleoclimatic indicators. Physical properties of lake water are caused by the temperature dependent isotopic fractionations during chemical reactions. The oxygen isotopic has been used for many years to study the climate and hydrological cycle. The interaction between water and its surrounding environment can be preserved for ages and can serve as an indicator record of previous climate change (Hansen, 2011). Paleoclimatic records from peat bogs, ice sheets, and lake and ocean sediments provide a mosaic of sufficient documented regional responses to global climate changes. The timing of these records should be accurate (Odezulu, 2011). Differences in response time for different records should be considered during interpretation. Isotopes, therefore, serve as sufficient paleoclimate indicators due to their accurate interpretation of the surrounding environment and the immediate climate. Diatoms may only be useful in providing information about previous climatic conditions. This is because of their static nature. To provide recent information about precipitation patterns, oxygen and carbon isotopes provide reliable information due to their ability to respond to different climatic conditions. A drop in isotopic ratios denotes effects caused by warming processes (Könitzer, Leng, Davies, Stephenson, 2012). From the above analysis, it is evident that stable and high precision isotopes are indicators of climatic change. 5.0 REFERENCES Aggarwal, P. K., Froehlich, K. F., & Gat, J. R. (2005). Isotopes in the Water Cycle Past, Present and Future of a Developing Science. Dordrecht: International Atomic Energy Agency (IAEA). Battarbee, R. W., & Binney, H. A. (2008). Natural climate variability and global warming: a Holocene perspective. Malden, MA: Blackwell Pub.. Dawson, T. E. (2007). Stable isotopes as indicators of ecological change. London: Academic Press. Erbs-Hansen, D. R., Knudsen, K. L., Olsen, J., Lykke-Andersen, H., Underbjerg, J. A., Sha, L. (2013). "Paleoceanographical development off Sisimiut, West Greenland, during the mid and late Holocene: A multiproxy study." Marine Micropaleontology, 102, 79–97 Gerhard, L. C., Harrison, W. E., & Hanson, B. M. (2001). Geological perspectives of global climate change. Tulsa, OK: American Association of Petroleum Geologists in collaboration with the Kansas Geological Survey and the AAPG Division of Environmental Geosciences. Ghosh, P., & Brand, W. (2003). Stable isotope ratio mass spectronomy in global climate change research. International journal of mass spectronomy, 2, 1-33. Hansen, D. R. (2011). Paleoecology of North Atlantic high-resolution sites during the holocene: the Skagerrak-Kattegat and West Greenland shelf : PhD thesis. Aarhus: Department of Earth Sciences, Aarhus University. International conference, arctic roads operating, maintaining and building roads in a climatically challenging environment : Sisimiut 3 - 5 March 2007. (2007). Sisimiut, Greenland: Sanaartornermik Ilinniarfik ;. Isotope Hydrology a Study Of The Water Cycle.. (2010). London: Frame publications. Kemp, A. C., Vane, C. H., Horton, B. P., Culver, S. J. (2010). "Stable carbon isotopes as potential sea-level indicators in salt marshes, North Carolina, USA." The Holocene, 20, (4) 623–636 Könitzer, S. F., Leng, M. J., Davies, S. J., Stephenson, M. H. (2012). "An assessment of geochemical preparation methods prior to organic carbon concentration and carbon isotope ratio analyses of fine-grained sedimentary rocksKönitzer." American Geophysical Union 1-12. Leng, M. J. (2006). Isotopes in palaeoenvironmental research. Dordrecht: Springer. Leng, M., & Anderson, J. (2003). Isotopic variations in modern lake waters from western Greenland. The Holocene, 4, 605-611. Leng, M. J., & Anderson, N. J. (2014). "The influence of Holocene climate and catchment ontogeny on organic carbon cycling in low-Arctic lakes of SW Greenland." EGU General Assembly 2014. Vienna: Geophysical Research Abstracts. Vol. 16, EGU2014-2929 Mackay, A. (2005). Global change in the holocene. London: Hodder Arnold. Odezulu, C. I. (2011). Stable hydrogen and oxygen isotopic variations in natural waters in North Florida implications for hydrological and paleoclimatic studies. Tallahassee, Fla.: Florida State University. Seidov, D., Haupt, B. J., & Maslin, M. (2001). The oceans and rapid climate change: past, present, and future. Washington, D.C.: American Geophysical Union. Taniguchi, M., & Holman, I. P. (2010). Groundwater response to a changing climate. Boca Raton [Fla.: CRC Press. Volkman, J. K. (2006). Marine Organic Matter Biomarkers, Isotopes and DNA. Berlin Heidelberg: Springer-Verlag.. Zimmerman, D. J. (2001). Isotopes. London: Frame Publications. Read More