Abstract:
A fundamental goal of the Earth Science community is to understand how perturbations on Earth’s surface are recorded in the stratigraphic record. Recent Source to Sink (S2S) studies of the Waipaoa Sedimentary System (WSS) are synthesized to provide a holistic perspective of the processes that generate, transport and preserve sedimentary strata and organic carbon on the Waipaoa margin in the late Quaternary. Rapid uplift associated with subduction processes and weak sedimentary units have conspired to generate rapid rates of incision and erosion in the Waipaoa catchment since the Last Glacial Maximum (LGM). We show that although much of the sediment exported offshore during this time interval originated from valley excavation, a substantial portion emanated from hillslopes, mostly through deep-seated landslide and earthflow complexes. Lacustrine sediments deposited in naturally-dammed 7-ky-old Lake Tutira provide a record of Holocene environmental controls on upper catchment sedimentation in the WSS. 1400 storms are identified, with one storm period (1830–2030 cal.yr BP) of particularly high magnitude and frequency. Storm frequency is modulated by the waxing and waning of atmospheric teleconnections between the tropics and Antarctica. Furthermore, clear long-term changes in sediment yield are evident from the Lake Tutira record following human settlement as conversion to pasture is accompanied by a 3-fold increase in the long-term lake sedimentation rate.Whereas there is ample evidence that Waipaoa River flood deposits are routinely deposited in the sheltered confines of Poverty Bay, over the longer term, waves and currents subsequently resuspend and transport these deposits both onshore (coarse fraction) and offshore (fine fraction). The timing of flood sediment supply is more often driven by wave events that are not associated with river flooding. Therefore, we conclude that asynchronicity of sediment delivery by the river and wave resuspension and transport out of Poverty Bay in most instances precludes the direct preservation of flood events in the stratigraphic record of the Waipaoa Shelf.
Over the longer term, the sediment package preserved on the shelf and slope since the LGM can be explained in large measure by sequence stratigraphic models forced by rising to stable sea level and ongoing tectonic deformation of the margin.Sediment budget exercises that consider both modern (river discharge versus centennial accumulation rates) and post-LGM (terrestrial production versus offshore isopachs) mass balances indicate that about half of the total sediment production from the Waipaoa escapes the study area. Moreover, a coupled sediment transport-hydrodynamic shelf model and observations of textural trends on the shelf indicate that a large fraction of the sediment is carried outside the study area along the shelf to the northeast by the river plume or by combined current/wave activity. Therefore, we can conclude that the WSS, as originally defined, is an open system from the present day through the LGM.The organic matter associated with sediment as it moves from upland source to marine sink is a product of particle history providing a record of materials that have cycled over timescales of days to millions of years. The ubiquity of fossil Organic Carbon (OC) in both the terrestrial and marine realms of the Waipaoa attests both to the chronic nature of its source, crumbling mudstones further destabilized by landuse, and its biogeochemical recalcitrance. Modern OC derived from extant ecosystems, though reactive, persists by virtue of its continual production along the source-to-sink transect. The nature of the modern OC changes along the source-to-sink path, however as new sources are added and upstream sources are lost. The Waipaoa, like other small mountainous rivers on active margins, exhibits a high riverine OC preservation efficiency (>50%) in its marine depocenter because of the rapid, event-driven accumulation of sediment. Rapid burial below the dynamic sediment-water interface is protective of otherwise reactive OC. The Waipaoa contrasts with dispersal systems on wide, energetic shelves (e.g., the Amazon and Fly Rivers) where sediment is extensively refluxed in oxygenated overlying water resulting in the biogeochemical incineration of particulate OC.
