Warren W. Dickinson

Heavy metal history from cores in Wellington Harbour, New Zealand

Analysis of ten heavy metals (Ag, Cd, Cr, Cu, Fe, Mn, Ni, Pb, Sb, Zn) in six sediment cores from Wellington Harbour show both anthropogenic enrichments and diagenetic modifications. Absolute concentrations determined by two methods, x-ray fluorescence and acid leaching for bioavailability, are not comparable. However, vertical trends in concentrations of the cored sediment are comparable. To assess levels of anthropogenic pollution, enrichment factors (enriched concentrations in upper core divided by background levels in lower core) are preferred over index of accumulation (Igeo) values because preindustrial or background levels of heavy metals are well constrained. The ten metals are placed into three groups: (1) Cu, Pb, and Zn, which show the most anthropogenic enrichment; (2) As, Cd, Cr, Ni, and Sb, which are often associated with anthropogenic pollution but show only minor enrichment; and (3) Fe and Mn, which are diagenetically enriched. Assuming harbor waters are well mixed, anthropogenic enrichments of Cu, Pb, and Zn, are time correlative, but the degree of enrichment depends on the method of analysis and core location. Levels of As, Cd, Pb, and Zn show small variations in preindustrial sediments that are not related to changes in grain size and probably result from changes in the oxidation-reduction potential of the sediments and salinity of the pore waters.

Antarctic permafrost: An analogue for water and diagenetic minerals on mars

At high altitudes (>1000 m) throughout the Dry Valleys, Antarctica, liquid water is rare, yet ground ice, which may be millions of years old, is pervasive in glacial sediments and bedrock. The origin of this ice is different from its arctic and alpine counterparts and may be similar to water on Mars. We present chemical and isotopic analyses of Antarctic ground ice from cores of Sirius Group sediments at Table Mountain in the Dry Valleys. These data, together with the presence of diagenetic calcite and chabazite in the frozen sediments, indicate that the ice and minerals accumulated over long periods of time from atmospheric water vapor and brine films formed on the surface of the ground. This analogy indicates that ferric-bearing minerals could precipitate under present conditions in the Martian soil.

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.

The Diagenetic Role of Brittle Deformation in Compaction and Pressure Solution, Etjo Sandstone, Namibia

Scanned-cathodoluminescence (CL) imaging of the quartz-rich, porous Etjo Sandstone from northern Namibia shows that brittle deformation has played a major role in developing arcuate and interpenetrated grain contacts. Such contacts, previously interpreted to result from pressure solution, are seen in scanned-CL images to arise primarily from rearrangement of fragments formed by brittle deformation. Brittle deformation dominates compaction and produces extensive microfractures that heal with authigenic quartz cement. The volume of intragranular authigenic cement is significant and represents a previously unrecognized sink for silica in sandstones. True pressure solution is minor in the Etjo and is generally limited to contacts between brecciated fragments and unfractured, detrital grains. In addition to this pressure solution, silica may also be mobilized from the dissolution of comminuted fragments near grain contacts. However, the amount of silica imported into grains is substantially larger than that which appears to come from dissolution sites. Grain overlap can no longer be considered to arise from simple pressure solution, and the volume of authigenic quartz measured in sandstones must include intragranular fracture-filling cement as well as overgrowths and pore-filling cement.

AUTHIGENIC CHABAZITE AND IMPLICATIONS FOR WEATHERING IN SIRIUS GROUP DIAMICTITE, TABLE MOUNTAIN, DRY VALLEYS, ANTARCTICA

ABSTRACT Petrographic examination shows authigenic chabazite and calcite in the ice-free or weathering horizon of glacially deposited diamictites in the Sirius Group on Table Mountain in the Dry Valleys area of Antarctica. Some samples contain as much as 18% chabazite, which fills pores in matrix clays. These pores are thought to have formed by repeated cycles of freezing and thawing during a warmer and wetter climate than the present. The chabazite is sodium-rich and has an average composition of (Ca0.9Na1.9K0.2)Al3.9Si8O24·6H2O. The presence of authigenic minerals in the Sirius suggests that chemical weathering is active and may take place in frozen ground along interfacial films of brine. Precipitation of the minerals is likely to occur at the boundary between the ice-free and ice-cemented horizons. As the ice cement sublimates, the film of brine becomes more concentrated until precipitation occurs in the process of efflorescence. Precipitation is kinetically driven by sublimation, allowing the crystals to grow in the open pore space of the ice-free horizon just above the ice cement. Because the depth to the ice-cement boundary fluctuates in response to major climatic changes, authigenic minerals that mark the position of this boundary may be a proxy indicator of past climates.

Depth and seasonal variations in the thermal properties of Antarctic Dry Valley permafrost from temperature time series analysis

[1] We present the first dedicated study of the thermal properties of perennially frozen, ice-cemented, subsurface Dry Valley permafrost. From time series analysis of 14 months’ temperature measurements, we resolve depth and seasonal variations in the thermal properties at two nearby sites at Table Mountain with different origin, composition, and polygonal ground patterning. We determine apparent thermal diffusivity (ATD) profiles directly from thermistor array measurements at 13.5-cm-depth intervals and 4-hour time intervals in the top 2 m. We treat the system as purely conductive year round due to the cold temperatures and compare the performance of several common analysis schemes with a graphical finite difference method that we present in detail. This comparison is facilitated by one site showing strong depth variations including an abrupt twofold increase in ATD across a sharp compositional boundary. We characterize the composition of the inhomogeneous ground from recovered cores and estimate an ice-fractiondependent heat capacity in the range C = 1.7 ± 0.1 to 1.8 ± 0.1 MJ m � 3 � C � 1 . We calculate apparent thermal conductivity profiles that correlate very well with the core compositions. The conductivity generally lies in the range 2.5 ± 0.5 W m � 1 � C � 1 but is as high as 4.1 ± 0.4 W m � 1 � C � 1 for a quartose Sirius sandstone unit at one site. The seasonal variation in the ATD is consistent with its expected temperature dependence. INDEX TERMS: 3299 Mathematical Geophysics: General or miscellaneous; 5134 Physical Properties of Rocks: Thermal properties; 5194 Physical Properties of Rocks: Instruments and techniques; KEYWORDS: Dry Valleys, Antarctica, thermal properties of frozen ground, permafrost

Discriminating depositional environments of sands from modern source terranes using modal analysis

Abstract Composition of modern beach, dune and fluvial sands from the Gulf of Mexico coast, and the Kapiti and Foxton Coasts, New Zealand, were compared in order to establish petrological criteria that discriminate between depositional environments and subenvironments. The three areas were chosen because of their different sand compositions. Sand from the Gulf of Mexico is quartzofeldspathic to feldspatholithic. Sand from the Kapiti Coast sands is feldspatholithic and sand from the Foxton is lithofeldspathic. Using a confidence level of 95%, petrographic data were plotted as bar charts for each environment (beach, dune, river) and for each subenvironment (inshore, foreshore, backshore of the beaches, stoss, crest, lee face of eolian dunes, upstream and mouths of the river). The best discrimination of beach, dune and river environments was achieved for the Gulf of Mexico sands using Q – F – L and Qm – K – P plots. Terranes with multiple source rock types such as the Gulf of Mexico enhance petrological discrimination between sands from beach, dune and river subenvironments and environments. In terranes with few rock source types such as the Kapiti Coast, only the river sands can be discriminated from those of other environments. The single rock source of the Foxton Coast allows little petrological discrimination between the beach, dune and river sands.This study examines compositional differences between beach, dune and river subenvironments and environments, and the degree to which these differences are dependent on the variation of rock types in the source terrane. The study also shows the usefulness of Q – F – L , Qm – K – P and Lv – Lm – Ls bar charts. The discrimination among beach, dune and river sands is partly due to grain-size variation.

Petrography of quartz grains in beach and dune sands of Northland, North Island, New Zealand

Abstract Petrographic and scanning electron microscopy (SEM) analyses of quartz grains from beach and dune sands were carried out in the western and eastern Northland coasts, New Zealand, to examine variations in durability and surface texture, which are controlled by mechanical and chemical processes, in profiles across beach and dune environments. This was done through point counts of quartz grain properties based on extinction angle and crystallinity. Variations in surface texture were assessed through SEM observations of mechanical features (conchoidal fractures, smooth surfaces, groove forms) and chemical features (solution pits, etching, silica deposits). Mechanically produced grooves are associated with beach sands affected by the high energy of the surf zone. Both mechanical and chemical processes occur in the eastern dune sands. They are associated with the greater abundance of angular grains in the eastern dune sands than the western dune sands. In addition, conchoidal fractures produced by the collision of grains in aeolian environments and linear and curved grooves produced by quartz grains from the beach support the mechanical processes taking place in the dunes. Solution pits, etching, and the presence of diatoms in the quartz grains are associated with pedogenesis and high silica precipitation in the eastern beach and dune sands. The durability of coarse‐grained polycrystalline quartz relative to fine‐grained polycrystalline quartz suggests that chemical abrasion exerts control over the distribution of quartz types in the dune sands.

Rapid soil accumulation in a frozen landscape

Polygonal patterned ground is a dominant geomorphic feature in the McMurdo Dry Valleys, Antarctica. In central Beacon Valley, polygons are considered to be ancient expressions of stagnant surface processes and are thought to be stable for millions of years. However, the actual rate at which these polygons form and alter is not known. We describe a detailed cross section of a polygon, make several depth profiles of meteoric 10Be along this cross section, and use optically stimulated luminescence to date a few key samples at the center of this polygon. While confirming conclusions of previous studies that the polygon shoulders are stable on a 100 k.y. time scale and experience little vertical sediment mixing, our results also give clear evidence of eolian transport to the polygon center, leading to a sediment accumulation rate of 3 cm k.y.−1 over the past 15 k.y. Moreover, the data suggest that the accumulated material is locally derived, and, hence, surface erosion of the polygon shoulders must exist that cannot be recorded by meteoric 10Be. We conclude that polygon modification and soil accumulation under the apparent frozen conditions of Beacon Valley is an active and ongoing process.

Depositional environment of Sirius Group sediments, Table mountain, dry Valleys area, Antarctica

Outcrops and cores of the Sirius Group sediments were studied at Table Mountain, Dry Valleys area, Antarctica. These sediments form a surficial veneer at least 9.5 m thick. Three facies — a gravelly sandstone, a sandstone, and a sandy conglomerate — are mapped and described from 13 outcrops and three cores. The gravelly sandstone, constituting 13%of all cored material, is bimodal with matrix‐supported clasts comprising 5–33%of the facies. Fabric analysis indicates that it was deposited primarily by lodgment from glacial ice but with minor elements of meltout and flow. The sandstone facies, constituting 77%of all cored material, is a well‐sorted, fine‐ to medium‐grained sand, which commonly has laminated bedding. It is predominantly a glaciofluvial deposit but has some glaciolacustrine elements. The sandy conglomerate, constituting 10%of all cored material, is a minor facies. It is massive and clast‐supported. It was deposited in a high‐energy environment suggestive of subglacial meltwater channels. Sirius Group sediments at Table Mountain are the result of wet‐based ice advancing and retreating over waterlain deposits. This is consistent with an advancing ice mass in climatic conditions that were warmer than present. The majority of the sediments were deposited by alpine ice following a similar pathway to the present‐day Ferrar Glacier and as such the depositional environment is one that concurs with evidence of a stable East Antarctic Ice Sheet approach. At Table Mountain, the predominantly glaciofluvial and glaciolacustrine facies is inferred to represent a more distal part of the Sirius Group environment than that seen at other outcrops in the Dry Valleys.