Mind the gap: an absence of valley-fill deposits identifying the Holocene hypsithermal period of enhanced flow regime in southeastern Australia

The Holocene sedimentary record in southeastern Australia is present in a range of landscape settings, such as upland swamps (dells), internally drained lake basins, alluvial fans, and mid-catchment and lowland floodplains. An assessment of the best-constrained basal radiocarbon dates in valley-fill locations between 30 and 42°S and upstream of last-glacial eustatic influences yields an intriguing pattern. The record for fluvial sites with catchment areas >30 km² exhibits a distinct gap in the alluvial record between 8 and 4 ka BP (10-4.5 ka). In contrast, data for eleven upland-swamp sites with catchment areas <50 km² exhibit a broader spectrum of basal ages, albeit with some reduction of activity during the alluvial gap. We suggest that the period 8-4 ka BP in the sedimentary record at the fluvial sites reflects the early to mid-Holocene climatic optimum independently recognized in proxy climate data in the region. It was a period of enhanced water discharges, stable well-vegetated catchments and low sediment yields, and therefore greatly limited sediment sequestration, and it has been termed the Nambucca Phase. In upland swamps, however, threshold-driven processes produce an episodic landscape responses during much of the Holocene. Contrasting results in upland compared with middle and lower basin locations demonstrate the non-uniform landscape response to climatic changes during the Holocene in southeastern Australia.