John L. Isbell

High-Precision U-Pb Calibration of Carboniferous Glaciation and Climate History, Paganzo Group, NW Argentina

The duration and geographic extent of Carboniferous glacial events in southern Gondwana remain poorly constrained despite recent evidence for a more dynamic glacial history than previously considered. We report 10 high-precision (2! ± <0.1%) U-Pb ages for the Permian-Carboniferous Paganzo Group, NW Argentina, that redefi ne the chronostratigraphy of the late Paleozoic Paganzo and Rio Blanco Basins, and signifi cantly refi ne the timing of glacial events and climate shifts in the western region of southern Gondwana. Radiometric calibration of the Paganzo Group indicates three pulses of Carboniferous glaciation in the mid-Visean, the late Serpukhovian to earliest Bashkirian, and between the latest Bashkirian to early Moscovian. An abrupt shift in depositional style from high-sinuosity single-storied fl uvial deposits and clay-rich paleosols to low-sinuosity multi storied feldspathic fluvial deposits inter calated with eolianites and calcic paleosols is constrained to the latest Moscovian and earliest Kasimovian. These constraints indicate a relatively abrupt climate shift from humid-subhumid to nonseasonal semiarid regional climate conditions that occurred signifi cantly earlier than previously inferred (Early Permian). This period of high-latitude aridity was contemporaneous with a shift to dryland depositional environments and a major vegetation regime shift documented throughout the Pangean paleotropics in the Pennsylvanian.

Reevaluation of the timing and extent of late Paleozoic glaciation in Gondwana: Role of the Transantarctic Mountains

Evidence from Antarctica indicates that a 2000-km-long section of the Transantarctic Mountains—including Victoria Land, the Darwin Glacier region, and the central Transantarctic Mountains—was not located near the center of an enormous Car- boniferous to Early Permian ice sheet, as depicted in many paleo- geographic reconstructions. Weathering profiles and soft-sediment deformation immediately below the preglacial (pre-Permian) un- conformity suggest an absence of ice cover during the Carbonif- erous; otherwise, multiple glacial cycles would have destroyed these features. The occurrence of glaciotectonite, massive and strat- ified diamictite, thrust sheets, sandstones containing dewatering structures, and lonestone-bearing shales in southern Victoria Land and the Darwin Glacier region indicate that Permian sedimenta- tion occurred in ice-marginal, periglacial, and/or glaciomarine set- tings. No evidence was found that indicates the Transantarctic Mountains were near a glacial spreading center during the late Paleozoic. Although these findings do not negate Carboniferous glaciation in Antarctica, they do indicate that Gondwanan glacia- tion was less widespread, and, therefore, that glacially driven changes to other Earth systems (i.e., glacioeustatic fluctuations, cli- mate) were much smaller than previously hypothesized.

Tetrapod and Large Burrows of Uncertain Origin in Triassic High Paleolatitude Floodplain Deposits, Antarctica

Abstract Two types of large diameter burrows, recognized by non-overlapping size distributions, occur in high paleolatitude floodplain deposits of the Lower Triassic Fremouw Formation, Shackleton Glacier area, Antarctica. Type G (giant) burrows are gently dipping tunnels 8 to 19 cm in diameter. Type L (large) burrows are 2 to 6.5 cm in diameter, curved or subhorizontal tunnels that rarely branch; scratch markings on both burrow types generally are parallel or tangential to the long axis of the burrows. Type G burrows are interpreted as produced by tetrapods based on similarity in size, architecture, and surface markings to Permian burrows from South Africa that contain complete skeletons of therapsids. These are the first tetrapod burrows described from Antarctica. Type L burrows have characteristics of both fossil tetrapod and crayfish burrows, precluding identification of an unique producer. Triassic tetrapods, including therapsids, that lived in high southern latitudes probably burrowed to dampen the effects of seasonal environmental fluctuations, just as do many of their mammalian counterparts living today in high latitudes. The paleolatitudinal and paleooclimatic distributions of burrowing therapsids and their mammalian descendents can be assessed by focusing search efforts on very large burrows, and by identifying producers using criteria delineated herein; this will clarify the extent to which the burrowing habit originated and persisted in high latitudes.

Tetrapod Burrows From the Triassic Of Antarctica

Abstract Two new tetrapod trace fossil records from the Triassic of Antarctica are described. The first is a giant terminal chamber collected from the lower Fremouw Formation (Lower Triassic) at Wahl Glacier (Beardmore Glacier region, central Transantarctic Mountains). Comparison to South African burrows known to contain the cynodont Thrinaxodon liorhinus, suggests that the Antarctic fossil was created by a tetrapod of similar size. The second set of tetrapod burrow casts was collected from Member ‘A’ of the Lashly Formation (lower Middle Triassic) and, if correctly interpreted, represent the first evidence of tetrapods from the Middle Triassic of Victoria Land. Our findings demonstrate that tetrapods were present in Victoria Land during the Middle Triassic, despite not being recovered as body fossils until the Upper Triassic in that region. Comparison to morphologically similar South African trace fossils suggests that procolophonids might have produced the Antarctic burrows, but this attribution is necessarily speculative because burrow inhabitants have not been found in situ and no morphological feature uniquely ties procolophonids to this type of trace fossil. We propose that underground burrows and dens were important shelters for Antarctic terrestrial vertebrates during the Triassic, despite a relatively moderate climate.

Early Paleozoic sedimentation, magmatism, and deformation in the Pensacola Mountains, Antarctica: The significance of the Ross orogeny

Combined sedimentological, structural, and geochemical studies of a lower Paleozoic succession within the Pensacola Mountains, Antarctica, suggest that it probably formed in a foreland basin setting during the Ross-Delamerian orogen, a complex early Paleozoic convergent margin of Antarctica and Australia. The lower Paleozoic succession lies unconformably on a deformed(?) Neoproterozoic sequence (referred to here as Sequence 1) and is divided into three unconformity-bounded sequences (Sequences 2–4). The oldest sequence, Sequence 2, comprises Middle–Upper Cambrian platformal limestone (Nelson Limestone) and overlying Lower Ordovician silicic volcanic rocks of the Gambacorta Formation (U-Pb zircon age of 501 ± 3 Ma). The volcanic rocks crystallized from a high-temperature anhydrous magma derived from a lower crustal igneous source and may represent magmatism on the inboard side of a magmatic arc now largely absent from this part of the margin. Sequence 3 (Wiens Formation), in part conformable with Sequence 2, represents deposition by unconfined ephemeral streams followed by a marine transgressive unit. The base of Sequence 4 (Neptune Group) is a major erosion surface marked by karstification of the exposed Nelson Limestone and by calcrete pedogenesis. The Neptune Group is an alluvial fan complex typical of many syn- and post-orogenic red beds. The predominance of nonmarine and shallow marine sequences, and the facies and paleocurrent directions within the basin, suggest that it may be more typical of a “piggyback” basin than of a foredeep basin, with the alluvial fan complexes derived from advancing thrust sheets. Growth folds, progressive unconformities, and deformed clasts of underlying strata within basal conglomerates are consistent with active deformation during sedimentation and the proposed tectonic setting. The presence of variably plunging folds, some of which are transected by a slaty cleavage, suggests that deformation was in an oblique-slip setting perhaps due to oblique convergence along this part of the Antarctic margin during the Ross-Delamerian orogeny.

Paleozoic-Mesozoic crayfish from Antarctica: Earliest evidence of freshwater decapod crustaceans

Discovery of an Early Permian claw from Antarctica extends the fossil record of crayfish by ∼65 m.y. and demonstrates that decapod crustaceans had radiated into freshwater habitats by the late Paleozoic. Burrows in Lower Triassic rocks of Antarctica are among the oldest apparently constructed by crayfish. Their morphology is similar to modern crayfish burrows, and this demonstrates that burrowing behavior was established early in the evolution of this group. The new discoveries show that the earliest Permian crayfish were distributed in high paleolatitudes of southernmost Pangea, where they lived in freshwater lakes fed by glacial meltwater. Modern crayfish habitat, used as a guide to crayfish temperature tolerance, indicates that summer temperatures of streams and lakes near the South Pole that supported the crayfish probably reached 10–20 °C during Permian-Triassic interglacial intervals.

Depositional framework of Permian coal-bearing strata, southern Victoria Land, Antarctica

Abstract The Permian Weller Coal Measures of Antarctica are exposed in a narrow belt that extends along the edge of the Polar Plateau from the Mawson Glacier to the Mulock Glacier in southern Victoria Land. These rocks record a rapid change from glacial to postglacial conditions, with the establishment of polar peat forming conditions, during the late Paleozoic in Antarctica. The 222 m thick Weller consists of interstratified fine- to coarse-grained quartzo-feldspathic sandstone, laminated siltstone, mudstone and coal. Regional lithofacies patterns and paleocurrent orientations define a narrow elongate basin. In the lower Weller, coarse-grained sandstone sheets occur along both eastern and western basin margins. These deposits are similar to those of modern sandy braided streams. Toward the axis of the basin, this lithofacies grades into coarse-grained tabular sandstone. Lateral-accretion beds suggest that deposition of this lithofacies occurred within meandering streams. Coarse-grained basin-margin deposits interfinger with fine-grained basin-axis lithofacies of interstratified laminated siltstone and graded sandstone beds deposited from suspension and underflow currents within a lacustrine environment. In the upper Weller, braided stream deposits dominate much of the basin, grading downstream into meandering streams. Establishment of vegetation in southern Victoria Land occurred immediately following late Paleozoic deglaciation. Upward in the Weller, the distribution of plant fossils and coal seams is lithofacies dependent. Thick coals and abundant plant fossils occur in meandering stream lithofacies, while they are scarce within the lacustrine and braided stream deposits. Abrasion during transport probably contributed to the destruction of vegetable material in the braided stream environments, and to material introduced into the axial lake. Depositional patterns within the Weller Coal Measures are similar to those in the Takrouna Formation in northern Victoria Land. Similarities suggest deposition in either the same or a similar basin. Comparison of the Weller Coal Measures with Permian strata in the central Transantarctic Mountains suggest deposition in two separate basins, which are now juxtaposed across the Byrd Glacier, the sight of a possible Cenozoic strike-slip fault. Causes for the formation of the depositional basin containing the Weller Coal Measures are unknown; however, the geometry of the basin and the regional distribution of lithofacies are similar to those of rift basins and to basins excavated by glacial erosion. The ultimate cause of basin subsidence however, may be related to long-distance stress produced during late Paleozoic orogenic activity along the paleo-Pacific margin of Antarctica.

Permian polar forests: deciduousness and environmental variation.

Forests are expected to expand into northern polar latitudes in the next century. However, the impact of forests at high latitudes on climate and terrestrial biogeochemical cycling is poorly understood because such forests cannot be studied in the modern. This study presents forestry and geochemical analyses of three in situ fossil forests from Late Permian strata of Antarctica, which grew at polar latitudes. Stem size measurements and stump spacing measurements indicate significant differences in forest density and canopy structure that are related to the local depositional setting. For forests closest to fluvial systems, tree density appears to decrease as the forests mature, which is the opposite trend of self-thinning observed in modern forests. We speculate that a combination of tree mortality and high disturbance created low-density mature forests without understory vegetation near Late Permian river systems. Stable carbon isotopes measured from permineralized wood in these forests demonstrate two important points: (i) recently developed techniques of high-resolution carbon isotope studies of wood and mummified wood can be applied to permineralized wood, for which much of the original organic matter has been lost and (ii) that the fossil trees maintained a deciduous habit at polar latitudes during the Late Permian. The combination of paleobotanical, sedimentologic, and paleoforestry techniques provides an unrivaled examination of the function of polar forests in deep time; and the carbon isotope geochemistry supplements this work with subannual records of carbon fixation that allows for the quantitative analysis of deciduous versus evergreen habits and environmental parameters, for example, relative humidity.

Permo-Triassic arthropod trace fossils from the Beardmore Glacier area, central Transantarctic Mountains, Antarctica

Abstract Permian and Triassic lacustrine and fluvial-system deposits in the Beardmore Glacier area of the Transantarctic Mountains preserve a superb record of continental environments and evidence of life on extensive bedding plane exposures. They yielded the first invertebrate trackways reported from continental Permo-Triassic deposits of Antarctica, here assigned to the ichnogenera Diplichnites and Diplopodichnus, which were probably produced by myriapodous arthropods. A resting trace is compared to Orbiculichnus and interpreted as generated by a jumping insect. Plant life is represented by leaf impressions, fossil forests and peat, vertebrates by body and trace fossils, and invertebrate shallow infauna by near surface burrows. The small number and diversity of trackways recovered from the large bedding plane exposures suggest that trackway-producing arthropods were rare at these high southern palaeolatitudes.

Reconstruction off preglacial topography using a postglacial flooding surface; Upper Paleozoic deposits, Central Transantarctic Mountains, Antarctica

ABSTRACT Upper Paleozoic glacial deposits in the central Transantarctic Mountains are the basal deposits within a late Paleozoic to early Mesozoic basin that formed along the margin of the East Antarctic Craton. This basin was a foreland basin throughout much of its history, and was part of a larger-scale basin that stretched across the paleo-Pacific margin of Gondwanaland. Our reconstruction of the preglacial topography in the central Transantarctic Mountains suggests that deposition began within two topographic depressions located on top of eroded rocks of the lower Paleozoic Ross Orogenic Belt. Isopach, paleocurrent, and lithofacies data support such a hypothesis. This interpretation implies that tectonic activity was not a factor in the formation of the depositional basin, and that little r no tectonic activity was occurring along the adjacent continental margin during glacial deposition. Although glacial deposits are present all along the paleo-Pacific continental margin of Antarctica and Gondwanaland, our findings suggest that initial sedimentation began within a series of discontinuous depocenters. A flooding surface that separates upper Paleozoic glacial deposits from overlying postglacial black shales in the central Transantarctic Mountains is used as a datum for reconstructing the preglacial topography. The postglacial flooding surface resulted from flooding of the depositional basin following collapse of the Gondwanide ice sheet. Results using this approach aided us in reevaluating the factors that controlled the formation of the depositional basin in Antarctica and in determining the tectonic setting of the paleo-Pacific margin of Antarctica during deposition of the upper Paleozoic glacial rocks. The use of a flooding surface as a datum is a technique that may be useful for reconstructing paleotopography in other settings.