George R. Dix

HYDROTHERMAL VENTING WITHIN A CORAL REEF ECOSYSTEM, AMBITLE ISLAND, PAPUA NEW GUINEA

Shallow-water (5–10 m) hydrothermal venting in a nearshore coral reef environment at Ambitle Island in the Tabar-Feni island arc, east of Papua New Guinea, occurs as focused discharge of boiling fluid from discrete ports 10–15 cm in diameter, and as dispersed discharge of diffuse bubble streams that issue through the sandy mixed carbonate-volcaniclastic sea floor. Abiotic aragonite and microcrystalline ferroan, low-Mg calcite, interlaminated with Fe-oxyhydroxides, are the prominent hydrothermal precipitates. Geochemical attributes of aragonite (δ18O, δ13C, and fluid inclusions) suggest that cements formed from a solution with salinities <5‰ at temperatures of ∼100 °C, with probable contribution of hydrothermal CO2. Sr isotope ratios in abiotic (hydrothermal) aragonite (∼ 0.704 15) are similar to those in island-arc basalt and denote considerable subsurface water-rock interaction of meteoric water derived from the adjacent volcanic island. The Sr isotope ratio of a coral sample (0.707 46) collected adjacent to a vent portal suggests coral growth within a mixed seawater-hydrothermal environment.

New Archaeological Evidence for an Early Human Presence at Monte Verde, Chile

Questions surrounding the chronology, place, and character of the initial human colonization of the Americas are a long-standing focus of debate. Interdisciplinary debate continues over the timing of entry, the rapidity and direction of dispersion, the variety of human responses to diverse habitats, the criteria for evaluating the validity of early sites, and the differences and similarities between colonization in North and South America. Despite recent advances in our understanding of these issues, archaeology still faces challenges in defining interdisciplinary research problems, assessing the reliability of the data, and applying new interpretative models. As the debates and challenges continue, new studies take place and previous research reexamined. Here we discuss recent exploratory excavation at and interdisciplinary data from the Monte Verde area in Chile to further our understanding of the first peopling of the Americas. New evidence of stone artifacts, faunal remains, and burned areas suggests discrete horizons of ephemeral human activity in a sandur plain setting radiocarbon and luminescence dated between at least ~18,500 and 14,500 cal BP. Based on multiple lines of evidence, including sedimentary proxies and artifact analysis, we present the probable anthropogenic origins and wider implications of this evidence. In a non-glacial cold climate environment of the south-central Andes, which is challenging for human occupation and for the preservation of hunter-gatherer sites, these horizons provide insight into an earlier context of late Pleistocene human behavior in northern Patagonia.

Marine saline ponds as sedimentary archives of late Holocene climate and sea-level variation along a carbonate platform margin: Lee Stocking Island, Bahamas

A 1500-year, late Holocene history of coastal and lacustrine carbonate sedimentation is preserved in shallow ponds on Lee Stocking Island, Exuma Cays, Bahamas. Details of environmental change have been extracted by integrating lithostratigraphy, biostratigraphy (macrobiota, foraminifers, ostracodes), and chemical stratigraphy (C, O isotopes of foraminiferal and molluscan skeletal carbonate; MgO wt% of ostracode calcite) with a well defined 14 CA MS radiocarbon chronology. Carbonate deposition began within physically restricted, euryhaline coastal embayments, with several pronounced changes in salinity defined by biotic and calculated salinity variation (from MgO wt% in shells of Cyprideis americana). By about 700‐740 yr B.P., embayment closure occurred possibly related to changed longshore deposition associated with sea level rise and=or regional change in climate (previously documented). With closure, the initial euryhaline foraminifer assemblage was replaced by a predominant hypersaline biofacies (e.g., Triloculina sp.); with progressive basin fill, ostracode assemblages, calculated salinities, and variation in abundance of the gastropod Cerithidea sp. may resolve higher-order (and some extreme) salinity fluctuations throughout the remaining history of saline pond development. Foraminiferal isotope stratigraphy is compatible with that expected for hydrologically closed lake basins. Carbonate accumulation was effectively shut-down<200 years ago, replaced by stromatolitic growth. Present-day salinities vary according to water balance governed by rainfall and evaporation. A centuries-scale (300‐400 year) flux of abraded (reworked), marine-derived bioclasts, admixed with skeletal remains of indigenous biota, is also preserved in these ponds. Allochthonous sediment was transported by hurricane storm surges or related to abrupt transgressive events superimposed on an overall gradual rise in global sea level. We discuss evidence for both as controls on sedimentation. Our study illustrates that saline ponds on Bahamian islands are excellent sedimentary archives of local, regional, and possibly global paleoclimatic events of late Holocene age.

Systematic decrease of high δ13C values with burial in late Archaean (2.8 Ga) diagenetic dolomite: evidence for methanogenesis from the Crixás Greenstone Belt, Brazil

Abstract Remnants of Fe-poor (2700–4400 ppm) dolomite crystals, crystal aggregates, and microconcretions are cryptically distributed within stratiform bodies (tens of metres across) of metamorphic Fe-rich (3–6 wt% Fe) dolomite that occur in graphitic schists of the 2.8 Ga Ribeira˜o das Antas Formation in the Crixas Greenstone Belt, Brazil. Fe-poor dolomite is a diagenetic carbonate formed during burial of organic-bearing, muddy sediments. Using cathodoluminescence microscopy, a consistent sequence of dolomite growth stages is well defined within individual crystals, crystal aggregates, and microconcretions. From comparison with similar textures in Phanerozoic carbonates, we suggest that the early dolomite formed during progressive burial, aggregates having developed from mergent growth of individual crystals during mechanical and then chemical compaction. The δ13Cdolomite values of crystals and concretion cores are very high, 12–14‰ (PDB), and a negative shift of about 7‰ occurs over millimetres in subsequent dolomite generations. Early diagenetic Fe-poor dolomite and late-stage Fe-rich dolomite show overlapping ranges of initial 87Sr86Sr ratios, between 0.7080 and 0.7164, and δ18O values, −14 to −16‰ (PDB). These similarities may indicate isotopic exchange between the early dolomite and subsequent metamorphic fluids, yet the well preserved growth zonation of the Fe-poor dolomite suggests that such exchange did not follow a process of recrystallization. The decrease in the δ13C of dolomite is correlated with crystal and concretion growth during burial. This behaviour is interpreted to indicate a change in the δ13C of pore-water bicarbonate resulting from bacterially mediated organic diagenesis within the lower part of the zone of methanogenesis. The Crixas dolomite represents direct evidence that methanogenic bacteria were important contributors in shallow subsurface geochemical systems in the early Precambrian. The pattern of δ13C values found for the Crixas dolomite is also documented in other early to middle Precambrian dolomites and limestones. Previous interpretations have considered positive δ13C values to be the result of primary (oceanographic) controls, whereas variability in δ13C is commonly linked to metamorphic alteration. We suggest that the role played by organic diagenesis in controlling the δ13C values of diagenetic carbonates has been underestimated.

Patterns of Burial-and Tectonically Controlled Dolomitization in an Upper Devonian Fringing-reef Complex: Leduc Formation, Peace River Arch Area, Alberta, Canada

ABSTRACT Cumulative patterns of dolomitization in a subsurface Upper Devonian carbonate platform in the Peace River Arch area, Alberta, are related to progressive burial ( 1 km) fracturing focused basin-derived fluids along subvertical structural conduits and into adjacent permeable strata. In the Early Mississippian ( 3 wt % Fe) dolomite in the platform, heralding increased Fe mobilization during burial diagenesis. A negative shift in 18O values, -6.5 to -15, is associated with successive fracture-related dolomites, and is interpreted to reflect an increase in fluid temperatures. Fracture-related flow was probably rapid; petrographic and isotopic signatures of fracture-related dolomitization can be traced 300 m upsection from the Leduc Formation.

Paleokarst in the Lower Ordovician Beekmantown Group, Ottawa Embayment: Structural control inboard of the Appalachian orogen

Two paleokarsts, different in age, character, and origin, occur in dolostones of the Lower Ordovician Beekmantown Group of eastern Ontario, well within the interior of the Laurentian paleoplatform. Surficial to shallow subsurface ( p CO 2 and H 2 S concentrations arising from mixing of continentally derived pore waters with brines derived from dissolution of Beekmantown evaporites. Compared to the region9s geologic history, a pre–late Paleozoic age for formation of the regional paleokarst and mineralization is likely. The two platform-interior paleokarsts demonstrate unexpected links between tectonism, changes in paleohydrological patterns, and porosity development well inboard of the Appalachian orogen.

The geochemical record of hydrothermal mineralization and tectonism inboard of the Appalachian Orogen: the Ottawa Embayment

Abstract The history of mineralization of the Ottawa Embayment includes migration of basin brines within the confines of an Early Paleozoic basin, and cross-stratal fracture flow linked to structural reactivation of an underlying Neoproterozoic failed-rift system. Paleoporosity consists of paleokarst (PAL) in Lower Ordovician dolostone (Beekmantown Group), and fractures (now veins) that cross-cut both Lower Paleozoic (PL) and Precambrian (PC) strata. A series of contrasting mineral assemblages, which define changing fluid compositions, fill paleokarst cavities and rare interconnected paleofractures (now veins). First, there is an early-PAL succession of quartz, saddle dolomite, then calcite, with accessory hydrocarbon as well as sulphide and sulphate minerals. The quartz defines migration of saline (>20 wt.%) silica brines and hydrocarbon that spans a temperature increase (defined by fluid inclusions) from δ 13 C of the dolomite identifies strong sedimentary rock–water interaction, but a depleted 13 C composition (∼−14‰) for the calcite illustrates that, with cooling, paleokarst cavities were sites of bacterially mediated sulphate reduction. Remaining paleokarst porosity is filled with a late-PAL mineral assemblage that contains low-temperature (mostly

Stratigraphic Patterns of Deep-water Dolomite, Northeast Australia

ABSTRACT In ODP Hole 815A, located along the edge of the Townsville Trough and marginal to the Marion Plateau of northeast Australia, dolomite forms about 5-20 wt % of carbonate within a distal part of a muddy contourite succession of Pliocene (2.6-4.0 Ma) age. Integration of seismic, lithic, and biostratigraphic data identifies a history of current-controlled episodic deposition subsequently shut down during rapid subsidence by ~2.6 Ma. Overlying pelagic-periplatform sediment contains no or trace amounts of dolomite. High-order, rhythmic (1-5 m thick) variation in dolomite, quartz, and carbonate content within the contourite succession defines a past, high-order stratigraphic variation in diagenetic potential for dolomite. d13C values of dolomite reflect incorporation of at least two sources of bicarbonate mixed with seawater: (1) in more carbonate-rich sediment, d13C values (1 to -2.0%) indicate likely contribution from dissolution of metastable carbonate; (2) in more siliciclastic-rich sediment, with locally elevated total organic content, d13C values (-4 to -6%) reflect contribution from organic-matter diagenesis. The majority of d18O values of dolomite over the entire contourite succession vary between 3.2 and 2%, and are interpreted to reflect near-surface to shallow (tens of meters) subsurface, marine-derived diagenesis. A few dolomites with lower (2 to 0%) values are restricted to two stratigraphic intervals, and reflect one or some combination of the following processes: a second stage of dolomitization at greater depths; near-surface diagenesis with much warmer bottom waters; or burial diagenesis with mixture of meteoric- and marine-derived pore fluids. Magnesian calcite is absent today in the contourite succession. However, the depositional and diagenetic frameworks of the contourite succession suggest that metastability of aragonite and magnesian calcite, variable current activity, and seawater diffusion across the sediment-water interface all promoted dolomitization. Compilation of dolomite stratigraphy along slopes off northeast Australia illustrates that dolomite distribution in Hole 815A is one of four intervals (D1-D4) in strata younger than 4 Ma: D1, 0.3-0.8 Ma; D2, 1-1.3 Ma; D3 (D3a, ~ 1.8 and D3b, 2.0-2.4 Ma); and D4, 2.6-4.0 Ma. The stratigraphy is linked to temporal variation in supply of shelf- and pelagic-derived metastable carbonate regulated by changes in paleoceanography, eustasy, and tectonics. D1, D3b, and D4 are in sediments with ages similar to previously documented dolomitization stages in shallow-water platforms elsewhere in the world, and suggest a coincidence between elevated diagenetic potential for deep-water dolomitization in platform margins and substantive changes in ocean c volume transport linked to eustatic change in sea level.

The Great American Carbonate Bank in Eastern Canada: An Overview

The postrifted margin of Laurentia in eastern Canada had a rugged paleomorphology, with major salients and recesses formed during the long-lasting (Ediacaran to late Early Cambrian) breakup of Rodinia. After short-lived carbonate production during the Early Cambrian, the great American carbonate bank (GACB) was firmly established in the earliest Middle Cambrian as the last rift-related event (Hawke Bay event, late Early Cambrian), and was followed by mostly passive thermal subsidence of the continental crust of Laurentia. Middle to Upper Cambrian carbonates are well preserved in the Port au Port Group in western Newfoundland (St. Lawrence promontory). Scattered outcrops of upper Middle to Upper Cambrian sedimentary rocks are found in southern and eastern Quebec (Quebec reentrant), although most of the preserved Upper Cambrian facies in the reentrant consist of nearshore to fluvial clastics unconformably overlying the Grenvillian basement. The Cambrian shallow-marine carbonates are dominated by high-energy facies with significant thrombolite reefs at the platform margin. The succession consists of large-scale transgressive-regressive cycles known as Cambrian grand cycles. Some anomalies in stacking patterns are suggestive of local tectonic events that were hypothesized based on the nature (facies and age) of carbonate clasts that accumulated on the continental slope. The Cambrian–Ordovician transition occurred at a time of a major sea level lowstand that resulted in a significant unconformity in southern Quebec and Ontario. In western Newfoundland, this sea level fall is recorded in the regressive facies of the last Cambrian grand cycle but did not culminate in subaerial exposure. The duration of the depositional hiatus at the Cambrian–Ordovician transition increases toward the west from an early Skullrockian gap in the Philipsburg thrust slice in southeastern Quebec; the hiatus covered the entire Skullrockian in eastern Ontario. A major sea level rise at or near the base of the Ordovician resulted in sedimentation on an extensive peritidal, mud-dominated, low-energy carbonate platform. This platform is known as the St. George Group (western Newfoundland), the Beekmantown Group (southwestern Quebec and Ontario), the School House Hill Group (southeastern Quebec), and the Romaine Formation (Anticosti Island). The carbonate facies are characterized by large- and small-scale depositional cycles. Two third-order cycles are well documented in western Newfoundland. The presence of such cycles is also proposed farther south, although their precise character still has to be documented. Multiple fifth-order meter-scale peritidal-dominated cycles have been documented in the Lower Ordovician carbonates. A diachronous change in depositional style occurred along the margin of Laurentia near the base of the Middle Ordovician. Facies patterns became controlled by faulting and accumulation rates increased significantly. These changes occurred first in the late Ibexian in southeastern Quebec and in the early Whiterockian elsewhere. At most localities, this transition is also expressed in a significant subaerial unconformity that is recognized along the entire eastern (paleosouthern) margin of Laurentia. This subaerial event is interpreted as resulting from lithosphere upwarping in front of the migrating Taconic orogenic wedge. The west-directed migration of the tectonic peripheral bulge resulted in the final destruction of the GACB as sedimentation resumed in a tectonically active foreland basin.

Petrology and potential tectonic significance of a K-bentonite in a Taconian shale basin (eastern Ontario, Canada), northern Appalachians

A 6 cm thick K-bentonite, herein defined as the Russell Bed, occurs in an Upper Ordovician deep-basin shale succession in eastern Ontario, Canada, forming part of the distal Taconic foreland in eastern North America. The bed lies within the pygmaeus graptolite Biozone, which is about 451 to 452 Ma in age. Although some bentonites are reported from this interval in eastern North America, we are reporting the first set of compositional data for a bentonite of this age. Gamma-log correlation identifies a potential minimum distribution area of 5 km 2 for the K-bentonite, covering part of southern Quebec, New York State and eastern Ontario. The deposit coincides with the first influx of distal turbdites into this shale basin, associated with Taconic flysch, and simultaneous abrupt ventilation of the once anoxic deep-water basin, which had formed initially after foundering of the Upper Ordovician carbonate platform. Concurrent intrabasinal extinction of several graptolite species suggests that change in sedimentation, palaeoceanography and volcanism were linked to a regional external process. Compositionally, the bentonite is distinct from the older Ordovician platform deposits in eastern North America. The deposit contains abundant titaniferous phlogopite with 1.6% BaO, fluoroapatite with 2.5% F, and dynamically shaped glass spherules now altered to clay. The spherules and clay matrix constitute 45% of the bed and, compositionally, define an illite (> 90%)–smectite (I/S) structure with about 7.5% K 2 O%. Age-dating by Ar–Ar analysis of the phlogopite crystals yielded a younger than expected (440–445 Ma) age. This difference, along with evidence of localized chloritization of phlogopite, likely reflects known post-Ordovician hydrothermal activity within the basin. On the basis of several geochemical proxies, the magmatic source of the Russell K-bentonite falls within the trachyandesite field and was Ba-enriched. Comparison of geochemistry and mineralogy with older, Middle to Late Ordovician and younger Early Silurian K-bentonites within the Taconic orogen along eastern Laurentia and Baltica reveals that the potential source magma for the Russell Bed was more mafic, more alkaline, and less fractionated than sources typical of older (platform) bentonites. Instead, it is more similar to the younger Llandovery bentonites of Scandinavia and Scotland. It remains uncertain if it signals local or regional compositional change in volcanic source in the northern Appalachians.