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New Constraints On Formation Of Orthogneiss In Southern Adirondacks Using Integrated Analysis Methods Of Petrology, Zircon U-Pb Geochronology And Whole Rock GeochemistryThe Adirondack Mountains (ADK) form the southern part of the Grenville Province, a poly-deformed orogenic complex formed in the Mesoproterozoic during the formation of Rodinia. The ADK is subdivided into two domains, separated by the Carthage-Colton Mylonite Zone. The Lowlands are characterized by upper amphibolite-facies metasedimentary rocks, including marbles, evaporites, and volcanic units, whereas the Highlands are formed by granulite-facies meta igneous rocks and anorthosite-mangerite-charnockite-granite magmatic complex. A large suite of granulite-facies orthogneisses exposed in the Southern Highlands yields the oldest (>1.3 Ga) ages, however, those ages come from a limited number of outcrops. We present new petrological, geochemical, and geochronological results from several quartzofeldspatic orthogneiss units to better document the timing and tectonic setting of these rocks. All samples are mainly composed of quartz, felspar, biotite, pyroxene, and opaque minerals. Foliation and lineation are defined by elongated quartz ribbons and feldspar grains, or biotite laminae. High-temperature microstructures, such as flame perthites in feldspars or checkerboard extinction in quartz are commonly observed in thin sections. Major element analyses show that samples have calc-alkaline affinities, suggesting an arc environment. U-Pb zircon geochronology produced two age groups. 1150 Ma ages which are Shawinigan and 1350 Ma ages which are associated with pre-orogenic magmatism. Based on whole rock geochemistry the Shawinigan age rocks are associated with within plate magmatism and the 1350 Ma pre-orogenic magmatism is associated with magmatic arc activity.
High Resolution Record Of Ice Rafted Detritus In Central North Atlantic Deep-Sea Sediment Core V30-100High resolution records from North Atlantic deep-sea sediments have been instrumental in documenting millennial scale climate fluctuations during Marine Isotope Stages (MIS) 4-1, including variations in concentrations of ice rafted detritus (IRD) related to massive iceberg discharges from Northern Hemisphere ice sheets. We have analyzed 149 closely spaced samples from the top 2 meters of North Atlantic piston core V30-100 (44°06.5’N, 32°30’W) and produced detailed records of %IRD (%lithics=(#lithic grains >150 microns)/(#lithic grains >150 microns + #planktic foraminifera >150 microns)*100), IRD per gram (lithic grains >150 microns per gram sediment), and planktic forams per gram (whole planktic foraminifers >150 microns per gram sediment). %IRD reveals 4 distinct intervals of >80% IRD coupled with consistently very low forams per gram at 40-48cm, 80-88cm, 137-149cm, and 179-184cm separated by long intervals of significantly lower IRD (<20%). The IRD per gram (lithics per gram) is consistently above 3000 lithics per gram during the intervals of high %IRD but show considerably more variability. %IRD and lithics per gram show a distinct lack of correlation at higher values. This is likely due to the influence of changes in planktic foram production affecting our proxies of IRD input. Upon analysis of our high-resolution IRD records with the low-resolution carbonate record from V30-100 as well as our detailed comparison to other IRD records from the Central North Atlantic (Hemming, 2003) suggests that the 4 intervals of high %IRD in V30-100 correspond to well documented Heinrich events that occurred in the North Atlantic during the last glacial interval (MIS 4-2). We have correlated the high IRD event centered around 180cm to H5, and the interval centered around 145cm to H4. The IRD events centered around 85cm and 45cm have been correlated to H2 and H1, respectively. If these correlations are correct, H3 has not been recorded in V30-100.
Late Pleistocene To Middle Holocene Paleolimnological Variability Preserved In The Sediments Of Walden Pond, MA.After the Last Glacial Maximum, the climate of the Northern Hemisphere exhibited a general warming pattern until a period of time recognized as the Holocene Climatic Optimum. The trend of gradual warming was disrupted by multiple climatic events. Of these events, the most notable is the Younger Dryas. This is the cooling event that defines the end of the Pleistocene Epoch. The purpose of this study is to reconstruct paleolimnological environmental conditions in Eastern Massachusetts during the period of time bracketed by the Younger Dryas (YD) and the Holocene Climatic Optimum (HCO) and to test the hypothesis that these changes are sensitive to regional climate change. We utilized the sediment archival record preserved in Walden Pond (Concord, MA) for this study, specifically the proxy records of stable carbon, nitrogen, and sulfur isotopes. Initial sub-bottom SONAR data taken in 2016 indicated a thick sediment package in the deep western basin of the pond, and in 2017 a Livingstone sediment core recovered approximately 9 meters of sediment. Eleven AMS radiocarbon dates were used to calculate an age model of the core, and the model calculates a basal age of 13,775 ± 280 cal BP. Sediment samples taken at 2 cm intervals down the depth of the core reveal a relatively low %OC at the base of the core. This value is about 5.00% and starts drastically increasing to high of 18.00% OC at age 11,671 BP. This trend in increasing %OC matches with a display of an increasing sedimentation rate which occurs as we pull out of the Younger Dryas. With an increasing sedimentation rate and an increasing %OC, it becomes evident that there is an increase in productivity within water column. When comparing the sedimentation rate and the %OC to the δ13Cvalues become less depleted ranging from -28.00‰ to -23.00‰ between the base of the core up to 4000 yrs. BP with some fluxes in between. Increased productivity and poor mixing are both contributors to increased δ13C. Walden Pond is a contained environment with no input of water from rivers or streams. The main sediment source is surrounding matter, windblown sediment and precipitation. The Younger Dryas displays an evident signature in the data with abrupt changes in sedimentation rates and low percentages of organic carbon. Coming out of the cold, arid conditions of the YD and into relatively warmer temperatures, increased productivity and influx of organic matter can be expected. δ15N can also indicate rates of productivity, although the current data shows a stable curve with values remaining between 2.00‰ and 4.00‰.