James Shulmeister

Australasian evidence for mid-holocene climate change implies precessional control of Walker Circulation in the Pacific

Abstract At its simplest, the Pacific cell of the Walker Circulation may be regarded as a thermal circulation, driven by the temperature contrast between the west and east Pacific. Evidence from Australasia is summarized, indicating that this circulation was greatly weakened in the early Holocene but was enhanced at about 5000 BP. The initial effect of this enhancement was the intensification of the Northern Australian Monsoon, within the Australian tropics, but this was reversed, abruptly, after 3700 BP. The enhancement is attributed to an increased polar-equator pressure gradient after 5000 BP with consequent intensification of the overall circulation pattern in the southern hemisphere including the mid-latitude westerlies and the trade winds. This in turn, appears to be a result of increased southern hemisphere seasonality, driven by the precessional cycle. It is proposed that increased wind speeds off South America after 5000 BP, in both mid- and low-latitudes, enhanced upwelling and reduced sea surface temperatures (SSTs). This created a greater east–west surface temperature contrast across the Pacific, compounding a pre-existing trend towards enhanced circulation, and ‘flipping’ climates in the southern hemisphere Pacific Basin from an Early Holocene mode to a Late Holocene mode. Under reduced circulation conditions, Early Holocene climates at low latitudes in the Southern Hemisphere may have been dominated by monsoonal heat transfers.

A field based classification scheme for gravel beaches

A tripartite classification of gravel beaches, based upon morphodynamic properties, is proposed and demonstrated for 42 New Zealand beaches. The main advantage of this scheme is that it is based on simple visual classification that can be applied globally in the field and is underpinned by morphodynamic differences between the beach types. The three types identified from the results are: (1) pure gravel beach; (2) mixed sand and gravel beach; (3) composite gravel beach. Pure gravel beaches have steep slopes (tan β = 0.08-0,24) and gravels extending from the storm berm to below mean low water spring tide level. Mixed sand and gravel beaches have moderate slopes (tan β = 0.04-0.13) with sand and gravel entirely mixed both cross-shore and at depth. Composite gravel beaches have a steep gravel berm fronted by a low-angle intertidal terrace, with overall beach slopes of tan β = 0.05-0.14. On composite beaches there is distinct hydrodynamic cross-shore sorting of the sand and gravel component. These broad types are tested using discriminant analysis and the three classes are shown to be highly significant (F= 16.24, df=8, P<0.00005). The key discriminating variables are Iribarren number, beach width, average grain size and storm berm height.

Human occupation of northern Australia by 65,000 years ago

The time of arrival of people in Australia is an unresolved question. It is relevant to debates about when modern humans first dispersed out of Africa and when their descendants incorporated genetic material from Neanderthals, Denisovans and possibly other hominins. Humans have also been implicated in the extinction of Australia’s megafauna. Here we report the results of new excavations conducted at Madjedbebe, a rock shelter in northern Australia. Artefacts in primary depositional context are concentrated in three dense bands, with the stratigraphic integrity of the deposit demonstrated by artefact refits and by optical dating and other analyses of the sediments. Human occupation began around 65,000 years ago, with a distinctive stone tool assemblage including grinding stones, ground ochres, reflective additives and ground-edge hatchet heads. This evidence sets a new minimum age for the arrival of humans in Australia, the dispersal of modern humans out of Africa, and the subsequent interactions of modern humans with Neanderthals and Denisovans.

El Niño suppresses Antarctic warming

Here we present new isotope records derived from snow samples from the McMurdo Dry Valleys, Antarctica and re-analysis data of the European Centre for Medium-Range Weather Forecasts (ERA-40) to explain the connection between the warming of the Pacific sector of the Southern Ocean [ Jacka and Budd, 1998 ; Jacobs et al., 2002 ] and the current cooling of the terrestrial Ross Sea region [ Doran et al., 2002a ]. Our analysis confirms previous findings that the warming is linked to the El Nino Southern Oscillation (ENSO) [ Kwok and Comiso, 2002a , 2002b; Carleton, 2003 ; Ribera and Mann, 2003 ; Turner, 2004 ], and provides new evidence that the terrestrial cooling is caused by a simultaneous ENSO driven change in atmospheric circulation, sourced in the Amundsen Sea and West Antarctica.

River response to an active fold-and-thrust belt in a convergent margin setting, North Island, New Zealand

High-resolution digital elevation data (TOPSAR 10-m grid) are used to reconstruct Late Quaternary growth histories of subtle folding in the Wairarapa fold-and-thrust belt, North Island, New Zealand. Outcrop data of deformed latest Miocene and younger strata are combined with observations of warped and faulted late Quaternary terrace surfaces to unravel the geomorphic and structural history of the Huangarua River valley. Optically stimulated luminescence (OSL) dating of loess that accumulated on these strath terraces and paleosol stratigraphy allow temporal correlation among the terraces and with glacial climate cycles since the Last Interglaciation. These data indicate that five intervals of strath cutting occurred, at ∼ 125, 60-70, ∼ 30, ∼ 15, and <10 ka. Strath beveling is largely independent of local folding or regional base-level change. We hypothesize that straths are cut when an increase in sediment supply, during cool climatic periods, brings river sediment load and river transport capacity into balance. In the Wairarapa, strath-cutting events appear to occur near the end of cool climatic cycles. This study shows that the identification of subtle departures from regional topographic trends becomes practical when a high-resolution DEM is available. After subtraction of an average valley gradient from the digital topography, the residual topography on the terrace treads reveals cross-valley and longitudinal tilting. Although rates of folding are slow and the magnitude of deformation is commonly limited to less than a few tens of meters, these topographic anomalies define fold axes that coincide with subsurface structures. When combined with time control, these anomalies serve to define the patterns and rates of fold growth over the past 125 kyears.

Effects of debris on ice-surface melting rates: an experimental study

Here we report a laboratory study of the effects of debris thickness, diurnally cyclic radiation and rainfall on melt rates beneath rock-avalanche debris and sand (representing typical highly permeable supraglacial debris). Under continuous, steady-state radiation, sand cover >50mm thick delays the onset of ice-surface melting by >12 hours, but subsequent melting matches melt rates of a bare ice surface. Only when diurnal cycles of radiation are imposed does the debris reduce the longterm rate of ice melt beneath it. This is because debris >50 mm thick never reaches a steady-state heat flux, and heat acquired during the light part of the cycle is partially dissipated to the atmosphere during the nocturnal part of the cycle, thereby continuously reducing total heat flux to the ice surface underneath. The thicker the debris, the greater this effect. Rain advects heat from high-permeability supraglacial debris to the ice surface, thereby increasing ablation where thin, highly porous material covers the ice. In contrast, low-permeability rock-avalanche material slows water percolation, and heat transfer through the debris can cease when interstitial water freezes during the cold/night part of the cycle. This frozen interstitial water blocks heat advection to the ice-debris contact during the warm/day part of the cycle, thereby reducing overall ablation. The presence of metre-deep rock-avalanche debris over much of the ablation zone of a glacier can significantly affect the mass balance, and thus the motion, of a glacier. The length and thermal intensity of the diurnal cycle are important controls on ablation, and thus both geographical location and altitude significantly affect the impact of debris on glacial melting rates; the effect of debris cover is magnified at high altitude and in lower latitudes.

A Holocene Pollen record from Lowland Tropical Australia

A pollen record from a dunefield lake on Groote Eylandt, Northern Australia is presented. This is the first substantially complete Holocene terrestrial record from the seasonally humid lowland tropics of Northern Australia. The lake originated as a seasonal swamp prior to 10 000 BP. A progressive rise in water tables occurred until a permanent lake was established at about 9000 BP. From 9000 to 7500 BP the lake shows evidence of disruption in the surrounding dunefield. Prior to 7500 BP an open grassland covered the dunefield. After 7500 BP the area was rapidly colonized by Eucalyptus open forest and acacias. These types remain dominant to the present. The data suggest that conditions continuously ameliorated from the base of the record to the mid-Holocene and there is evidence of an effective precipitation maximum at about 4000 BP. Effective precipitation declined after 3800 BP but a recovery took place about 1000 BP.

Determining the origin and age of the Westland beech (Nothofagus) gap, New Zealand, using fungus beetle genetics

The formation and maintenance of the Nothofagus beech gap in the South Island, New Zealand, has been the focus of biogeographical debate since the 1920s. We examine the historical process of gap formation by investigating the population genetics of fungus beetles: Brachynopus scutellaris (Staphylinidae) inhabits logs and is absent from the beech gap, and Hisparonia hystrix (Nitidulidae) is contiguous through the gap and is found commonly on sooty mould growing on several plant species. Both species show distinctive northern and southern haplotype distributions while H. hystrix recolonized the gap as shown by definitive mixing. B. scutellaris shows two major haplotype clades with strong geographical concordance, and unlike H. hystrix, has clearly defined lineages that can be partitioned for molecular dating. Based on coalescence dating methods, disjunct lineages of B. scutellaris indicate that the gap was formed less than 200 000 years ago. Phylogenetic imprints from both species reveal similar patterns of population divergence corresponding to recent glacial cycles, favouring a glacial explanation for the origin of the gap. Post‐gap colonization by H. hystrix may have been facilitated by the spread of Leptospermum scoparium host trees to the area, and they may be better at dispersing than B. scutellaris which may be constrained by fungal host and/or microhabitat. The gap‐excluded species B. scutellaris is found in both beech and podocarp‐broadleaf forests flanking the Westland gap and its absence in the gap may be related to incomplete recolonization following glacial retreat. We also discuss species status and an ancient polymorphism within B. scutellaris.

A long late-Quaternary record from Lake Poukawa, Hawke's Bay, New Zealand

Abstract The Lake Poukawa Basin is a large co-seismic depression located at 20 m above mean sea level in Hawke’s Bay in eastern North Island, New Zealand. We present a detailed environmental history of the basin for the last c. 60 ka based on analyses of the top 105 m of a 200-m core record. Dating control is provided by radiocarbon, optically stimulated luminescence (OSL) and U/Th disequilibrium ages. The chronology is supported by nine tephras of inferred age including marker tephras, Kawakawa (22 590±230 yr BP at −18.25 m core datum), Tahuna (c. 35–43 ka) at −33.1 m core datum, and Rotoehu (45–50 ka) at −39.1 m core datum. Disagreements between some of the older tephra ages and the numerical ages from the OSL and U/Th dating mean that more than one age model can be applied. Three major lithostratigraphic units are identified: a basal calcareous silt with lignitic peats between 105.28 and 98.58 m of marine oxygen isotope stage (MIS) 3 age; an extended sequence of detrital shelly sands and silts, between 98.58 and 8 m of MIS 3 and 2 age; and a Holocene peat unit (MIS 1) from 8 to 0 m. Alternatively, but less likely, the basal unit may represent stage MIS 5a and the detrital shelly sands would then contain an amalgam of MIS 4, 3, and 2 deposits. We propose a notably moist phase represented by the peat which our numerical dating model places near the start of isotope stage 3. This suggests the existence of mild conditions during an interstadial in central New Zealand at c. 55–50 ka when a podocarp–beech–broadleaf forest of near-interglacial affinity surrounded the basin. The interstadial is marked by both lake and peat formation in the basin. After 50 ka a thermal decline set in, though the climate remained moist initially. Under these conditions, the Poukawa Basin was rapidly infilled by alluvial fan deposits from the surrounding hills. The floor of the basin was occupied by grasses and sedges, responding to both the highly disturbed environment and swampy conditions in the basin. After the deposition of the Rotoehu Ash, effective precipitation declined markedly and woody shrubs expanded across the previously swampy basin floor. The data suggest an apparent thermal decline of c. 6–7°C for much of MIS 2 and the latter half of MIS 3. The Holocene was marked by the establishment of fen and lake environments on the basin floor. Prior to human disturbance, podocarp–broadleaf forest surrounded the basin.

An analysis of phytolith assemblages for the quantitative reconstruction of late Quaternary environments of the Lower Taieri Plain, Otago, South Island, New Zealand I. Modern assemblages and transfer functions

An analysis of modern phytolith assemblages is presented.Phytolith assemblages were studied in modern surface soils and sediments of 28sites from east Otago, New Zealand, within a range of vegetation types andmicroclimates. No simple distinction could be made between vegetation types onthe basis of phytolith assemblage composition. A Principal Components Analysis(PCA) of the phytolith data set revealed that festucoid, chloridoid andspherical phytolith morphotypes formed strong associations with sites fromwetland, grassland, and forest vegetation types, respectively. Moreimportantly, a comparison of sample replicates from each field site using Squared ChordDistance (SCD) assemblage analysis showed that wetland and grassland sitestended to produce more internally consistent phytolith assemblages than forestsites. Environmental variables including pH, conductivity, altitude,precipitation and temperature were also gathered for each site. The ability ofeach environmental variable to reflect variance in the entire phytolithdata set was estimated by a series of Redundancy Analyses (RDA) with MonteCarlo permutation tests of statistical significance. After a forward selectionprocess, transfer functions were generated using Partial Least Squares (PLS)regression and calibration with jack-knife validation. The final transferfunctions have root mean squared errors of prediction for pH (0.47), logconductivity (0.38 μS cm), average annual precipitation (63mm), and average annual (0.28 °C), spring (0.38 °C) andautumn temperature (0.41 °C); the smallest group of environmental variablesexplaining the most variance in the modern phytolith data set. The most usefultransfer functions for application to fossil phytolith data andpaleoenvironmental interpretation are pH, log conductivity and annualprecipitation. The relationship between changes in pH and annual precipitationand phytolith assemblage composition found in this study presents aprima facie relationship with the potential to providedirect proxies for soil weathering and indirectly for paleoenvironmentalreconstruction.