Distinguishing Detrital and Authigenic Calcite From Transect Core Catcher Samples in Lake Junín, Peru

Lake Junín is one of the few lakes in the tropical Andes that predates the last glacial maximum. Exposure ages taken from end moraine boulders show that throughout Cenozoic glaciation, Lake Junín has not experienced glacial cover in the last ~1 million years, thus providing a continuous record of glacial and interglacial events through sediment influx. In 2015, an ~105-meter sediment core was taken from Lake Junín, and core catchers were obtained from each 3-meter core section. Percent CaCO3 and magnetic susceptibility analyses from the cores display a sequence of approximately seven glacial and interglacial cycles. Glacial periods are characterized by high magnetic susceptibility values (~98 SI units) and low percent CaCO3 (<25%), while interglacial periods are characterized by low magnetic susceptibility values (10 – 25 SI units) and significantly higher percent CaCO3 values (>60%). Samples containing high, medium, and low percent CaCO3 (>70%, 40%-70%, and 10%-40% respectively) were selected and sieved into three separate grainsize fractions, >53mm, >25mm <53mm, and <25mm to differentiate between detrital and authigenic calcite. Initial x-ray diffraction results of <25mm grainsize trend with a marginally higher peak intensity than the calcite standard (1228 I), with a d-spacing between 3.033 Å and 3.037 Å. Grainsizes that display peak intensities at a d-spacing 3.440 Å are interpreted to be dominant in while grainsizes that display peak intensities at a d-spacing of 2.880 Å are interpreted to be more magnesium-rich dolomite dominant. These results provide a means of discerning calcite origins in Lake Junín throughout glacial periodicity.