The Impact of Pleistocene Climate on Sediment Routing Systems: Reconstructing Sediment Dispersal from the Southern Alps to the Canterbury Continental Shelf, New Zealand

Abstract:

Climate and base level changes in the Pleistocene are expected to influence the relative storage and transfer of different size fractions in a sediment routing system. We study mechanisms of sediment routing and dispersal and their relative influence on marine strata formation during the Pleistocene using continental shelf strata drilled during IODP Expedition 317 to Canterbury Basin, South Island of New Zealand. We use semi-quantitative mineralogy from bulk, sand (2-0.063 mm), silt (63-15 μm), and clay (<2 μm) size fractions, to reconstruct sediment dispersal paths from the Southern Alps to the ocean. Silt is composed of lithic fragments and individual mineral grains, and the results are not informative on silt dispersal paths. Instead, it compares to bulk mineralogy, reflecting overall variations in terrigenous input to the shelf. Sand and clay mineralogy suggest the variable influence of two dispersal paths to Canterbury shelf: one reflecting direct input from the more proximal fluvial basin (Waitaki Basin), and another southern coastal source. A change in the sand quartz:plagioclase ratio from Middle to Late Pleistocene strata suggests a shift in the mode of sand dispersal from along margin (southern) to across margin (Waitaki). This likely resulted from relatively long sand residence time within the Waitaki Basin during the Middle Pleistocene, amplifying the influence of the southern source. This southern influence is supported by clay mineralogy that shows higher content of smectite versus illite+chlorite during this period. In contrast, Late Pleistocene sand and clay composition mostly reflect the dominant bedrock within the Waitaki basin indicating enhanced transfer of sediment from the Basin. We argue that this change in sediment dispersal is related to the expansion of alpine glaciers in the Southern Alps during the Middle Pleistocene that led to higher glacigenic sediment fluxes and shorter residence time within the sediment routing system.