Composition, age and origin of Pleistocene turbidite deposits at ODP site 1232, Nazca plate: Implications for volcanism and climate change in central south Chile

Abstract

A 371 m sequence of Pleistocene age turbidites was recovered at ODP (Ocean Drilling Program) Leg 202, Site 1232 at 39°53.45'on the Nazca Plate, seaward of the Peru-Chile Trench. Coarsest sediments within the turbidites (<300 μm) contain large amounts of fresh, angular volcanic glass shards. We investigated the variation in proportions of coarsest (>149 μm) glass, minerals and polymineralic grains, and microfossils downcore with the goal of discerning temporal changes and their underlying causes. Between 214 and 160 mbsf, the dominant coarse particle type changes from rounded, clay-rich granules, interpreted as altered pumice, upward to fresh, angular volcanic glass. This change correlates with an upward increase in turbidite thickness noted by Leg 202 scientists. Major and trace element analyses of individual glass shards show a geochemical signature consistent with a source within the adjacent Andean Southern Volcanic Zone. New calcareous nannofossil biostratigraphy brackets the sediment ages of the upper 288 m of the section to between 12.47 kyr and 275 kyr, which is consistent with published ages for the upper 97 m of the section and estimated maximum ages.Based on our dating of the section, the observed change in dominant coarse sediment type occurs between 225 and 185 kyr and correlates with a climatic transition from interglacial to glacial conditions. A plausible connection with climate is that the altered pumice grains formed by chemical weathering and fluvial transport under warm surface conditions, whereas the glass shards were transported by aeolian and glacial processes, leading to a lack of weathering and an angular texture. However, as a climate-related temporal change, this scenario conflicts with the absence of a similar change in sediment type during the subsequent Valdivian and Llanquihue interglacial-glacial transition starting at about 125kyr. Therefore, our conclusion is that, although the two coarse sediment types formed and were transported under different climate conditions, the temporal change is related to a shift in sediment source, supply or transportation pathway during the 185–225 kyr interval. A related possibility is that the onset of angular glass involvement in turbidity flows, during a glacial stage, marks the eruption of a new volcanic source or sources. Resolving these possibilities could be addressed with more detailed geochemical analysis.