XRD ANALYSIS OF SEDIMENT-CORE MATERIAL AS AN INDICATOR OF THE TRANSITION FROM VALLEY GLACIERS TO CIRQUE GLACIERS IN THE PERUVIAN CENTRAL CORDILLERA

Sediment provenance studies are an important tool in reconstructing glacial activity in recently glaciated landscapes. X-Ray Diffractometry (XRD) of sediment-core samples from the high Andes of Peru resolves retreat of the Suirucocha glacier from its valley position (~4,500m) to its current cirque position (~5,000m). Samples were taken from Lago Suirucocha, a high-altitude glacial lake, situated on the dry side of the Andes Mountains approximately 120km east of Lima, Peru. The geology of the area comprises mid- to high-grade metamorphic granitic gneisses and schists with Permian sandstones outcropping on the east side of the lake. These differences in geology permit provenance studies of the sediments – were they glacially or fluvially derived? By comparing the mineralogy of clay-sized-fraction sediment from four sites from/around the lake, a three-dimensional reconstruction of sediment influx can be determined. Livingston core samples were taken from a) two side valleys situated north and south of the lake and b) the palustrine section of the lake. A hammer core was taken from the lake bottom and a representative sample of the sandstone was taken from an outcrop. To determine the mineralogy of the core sediments, interval samples of approximately 10 grams were prepared for XRD using the methods of Moore (1989) and Huff (pers. comm). The use of the program PDXL allowed for mineral identification in each sample after XRD analysis. Results of mineral identification are represented by >75% bulk composition and can be divided into two provenance settings: a) zeolites and feldspars from proximal glacial valleys situated to the north, west and south and b) chlorite and quartz from the distal, non-glaciated sandstone valley. These indicator minerals suggest major shifts in sediment source and glacial activity stemming from a loss of Suirucocha glacier influence around 3,000 ¹⁴C years BP. The loss of glacier-melt discharge into the lake is noted in the centrally located, palustrine core by a loss of zeolites and feldspar sourced from the glacial valleys to an increase of chlorite and quartz sourced from the non-glaciated sandstone valley. This change in provenance of sediment influx into Lago Suirucocha helps resolve the transition from a valley-occupied glacier to the modern, cirque-dwelling ice.