Southernmost Argentina Research Articles

Palynofacies associated to hyperpycnite deposits of the Miocene, Cabo Viamonte Beds, Austral Basin, Argentina

The Miocene deposits in the Punta Basilica locality, southernmost Argentina, are included within the Cabo Viamonte Beds, Cabo Domingo Group, in the Austral foreland basin of Tierra del Fuego province. The prograding clinoform systems were accumulated during a weak compressional tectonic regime that allowed the development of a narrow shelf. Paleoenvironmental reconstructions suggest that these clinoforms comprise two dominant architectural elements, channel-levee and lobe complexes, formed mainly by density hyperpycnal currents in outer shelf to depositional slope environments. The transitional and recurrent (vertical and lateral) alternation between sedimentary structures without rheologic boundaries associated with the co-occurrence of plant remains (Nothofagus) are diagnostic criteria for the recognition of hyperpycnites. This type of density flow typically transports large volumes of sediment and organic matter from proximal to deep-marine settings. Four palynofacies types were recognized in a cluster analysis. In general, the palynofacies show predominance of spores and pollen grains, tissues, cuticles, and spongy to fibrous amorphous organic matter (plant and/or freshwater to brackish algae derived), which reflect different positions within the depositional system (e.g., levee-channel and lobe deposits). The co-occurrence of inshore (Batiacasphaera spp., Lingulodinium sp.) with relatively more oceanic (Operculodinium centrocarpum, Spiniferites spp.) dinoflagellates is a strong indication that shallow-water assemblages have been displaced into deep-water settings. Due to the presence of Lingulodinium hemicystum (first appearance data: 23.0 Ma.) and Pentadinium laticinctum (last appearance data: 11.6 Ma.) an age not older than Miocene and not younger than the Serravallian/Tortorian boundary for the Punta Basilica section is proposed.

Early to middle Holocene palaeoenvironmental reconstruction of the Beagle Channel (southernmost Argentina) based on terrestrial and marine palynomorphs

This paper presents the main palaeoenvironmental results obtained from a site located in the western sector of the Beagle Channel, Isla Grande de Tierra del Fuego, southernmost Argentina. The palynological analysis performed on marine sediments from Aserradero‐Lapataia 2 (latitude 54°51′22.7″S, longitude 68°34′22.8″W) allowed us to document the evolution of palaeoenvironmental conditions during the beginning of the Holocene marine transgression into the Beagle Channel. The results indicate that, prior to the marine incursion, the coastal areas were characterized by the presence of open‐grown shrubs and herbs along with woodland palaeocommunities. The aquatic environments were dominated exclusively by freshwater taxa. Around 8300 cal. a BP, seawater gradually flooded the channel while cold and high effective moisture conditions favoured the development of an arboreal vegetation with dominance of Nothofagus forest and scarcity of shrub and herbaceous communities. Gradually increasing salinities allowed the development of freshwater/marine transitional environments indicated by aquatic palynomorphs able to tolerate stressed conditions under fluctuating salinities. The increasing dinocyst diversification with dominance of heterotrophic taxa corroborates the establishment of a fully marine environment during the middle–late Holocene in the Beagle Channel.

Convicts and conservation: inmate labor, fires and forestry in southernmost Argentina

The modern penitentiary was rarely a fully closed, insular, or placeless institution. Approaching penology as environmental history can therefore restore connections that have been severed between prison studies, political ecology, and geography. This article uses the Ushuaia penal colony/penitentiary, located in southernmost Argentina, to explore relationships between penology, inmate labor, and forestry. In the early 1900s, inmates erected the prison that would house them, and labored to provide roads, government buildings, and electricity for Ushuaia. Their main activity was felling timber, which brought together the prison and forestry departments, thus linking discipline with deforestation, and blurring the lines between the interior and exterior of the penitentiary. After decades of extensive forest fires, the lack of a self-sustaining economy, and charges of inmate abuse, the prison was closed in 1947. A subsequent campaign to salvage the region by supplanting ominous images of a natural prison with those of a beautiful landscape resulted in the establishment of a national park in 1960. And, in 1997, fifty years after its closure, the defunct prison was converted to a museum. The national park and prison museum now attract thousands of tourists annually, offering two competing — rather than co-constitutive — versions of national history. This juxtaposition of green and dark tourism is not unique to Ushuaia, and thus highlights tensions inherent to when and where sites of punishment, conservation, and memory overlap.

Origin and significance of diagenetic concretions in sediments of Laguna Potrok Aike, southern Argentina

Authigenic minerals can form in the water column and sediments of lakes, either abiotically or mediated by biological activity. Such minerals have been used as paleosalinity and paleoproductivity indicators and reflect trophic state and early diagenetic conditions. They are also considered potential indicators of past and perhaps ongoing microbial activity within sediments. Authigenic concretions, including vivianite, were described in late glacial sediments of Laguna Potrok Aike, a maar lake in southernmost Argentina. Occurrence of iron phosphate implies specific phosphorus sorption behavior and a reducing environment, with methane present. Because organic matter content in these sediments was generally low during glacial times, there must have been alternative sources of phosphorus and biogenic methane. Identifying these sources can help define past trophic state of the lake and diagenetic processes in the sediments. We used scanning electron microscopy, phosphorus speciation in bulk sediment, pore water analyses, in situ ATP measurements, microbial cell counts, and measurements of methane content and its carbon isotope composition (δ13CCH4) to identify components of and processes in the sediment. The multiple approaches indicated that volcanic materials in the catchment are important suppliers of iron, sulfur and phosphorus. These elements influence primary productivity and play a role in microbial metabolism during early diagenesis. Authigenic processes led to the formation of pyrite framboids and revealed sulfate reduction. Anaerobic oxidation of methane and shifts in pore water ion concentration indicated microbial influence with depth. This study documents the presence of active microbes within the sediments and their relationship to changing environmental conditions. It also illustrates the substantial role played by microbes in the formation of Laguna Potrok Aike concretions. Thus, authigenic minerals can be used as biosignatures in these late Pleistocene maar sediments.

Major lake level fluctuations and climate changes for the past 16,000 years as reflected by diatoms and chironomids preserved in the sediment of Laguna Potrok Aike, southern Patagonia

Laguna Potrok Aike is a maar lake located in southernmost Argentina and is one of the few permanent lakes preserving a continuous climatic record from the semiarid Patagonian steppe. Furthermore, its location close to Antarctica provides a unique opportunity to compare paleoclimate from continental South America with the polar records. The analysis of subfossil chironomids and diatoms throughout a 16-m sedimentary record retrieved from this lake, combined with a well-constrained chronology of the last ca 16 ka BP, provided a high resolution paleoenvironmental reconstruction of the limnology of the lake and regional climate conditions. The combination of both bioproxies showed humid conditions during the Lateglacial, followed by drier conditions during the Holocene, resulting in large variations in lake level. Despite not showing a clear evidence of a cold reversal similar to the Antarctic Cold Reversal and/or the Younger Dryas, both records suggest high water levels and oligotrophic conditions between 16.4 and 11.5 cal. ka BP. The lake level drop that occurred at ca 8.7 ka BP is well documented by both bioproxies.

Provenance of eastern Magallanes foreland basin sediments: Heavy mineral analysis reveals Paleogene tectonic unroofing of the Fuegian Andes hinterland

Documenting variations in the composition of foreland basin detrital sediments is a useful tool for reconstructing the kinematic history of associated orogenic systems. Herein we report the results of our SEM-EDS heavy mineral analysis of the Late Cretaceous to early Miocene eastern Magallanes basin of southernmost Argentina. Our data indicate that Campanian to middle Eocene sediments had a mafic/ophiolitic provenance, which we interpret as being derived from the Patagonian–Fuegian magmatic arc and the mafic floor of the preceding Rocas Verdes marginal basin. Upper middle Eocene to lower Miocene heavy minerals, on the other hand, indicate a metamorphic/metasedimentary provenance, which we interpret as being derived from the Cordillera Darwin metamorphic complex. Together, these data indicate an abrupt shift in sediment provenance in middle to late Eocene time, thereby corroborating recent interpretations of the basin's detrital–zircon geochrononology, and providing further support for temporal and possibly genetic relationships between development of the Patagonian orocline, the opening of Drake Passage and the Oi-1 glaciation of Antarctica.

Assessment of land influence on a high-latitude marine coastal system: Tierra del Fuego, southernmost Argentina

The study deals with the determination of physico-chemical parameters, inorganic nutrients, particulate organic matter, and photosynthetic pigments on a monthly basis during an annual cycle from nine sampling sites of the coastal zone of a high-latitude ecosystem (Tierra del Fuego, Argentina). Nitrites and phosphates concentrations were similar to other systems of the south Atlantic coast (median, 0.30 and 1.02 μM, respectively), while nitrates were higher in all sampling periods (median, 45.37 μM), and silicates were significantly smaller (median, 7.76 μM). Chlorophyll a and phaeopigments have shown median values of 0.38 and 0.85 mg m(-3), respectively, while saturated values of dissolved oxygen were recorded throughout the study. The analysis reflected that nutrient enrichment seems to be linked to an anthropogenic source, the presence of peatlands areas, and a sink of Nothofagus pumilio woods. The area could be characterized in three zones related to (1) high urban influence, (2) natural inputs of freshwater, and (3) mixed inputs coming from moderate urban impacts.

Detrital-zircon geochronology of the eastern Magallanes foreland basin: Implications for Eocene kinematics of the northern Scotia Arc and Drake Passage

U/Pb detrital-zircon geochronology of eleven sandstones collected from Cretaceous through Oligocene strata of the eastern Magallanes foreland basin of southernmost Argentina records a dramatic provenance shift near the end of the middle Eocene at ca. 39 Ma. From the Late Cretaceous through most of the middle Eocene, detrital zircons reaching the foreland basin were dominantly contributed from ≤ 140 Ma sources, most likely derived from the Patagonian–Fuegian magmatic arc. In contrast, detrital-zircon populations of sampled upper Eocene and Oligocene strata are dominated by 150–190 Ma and pre-Mesozoic grains presumably derived from ignimbrites and granitoids associated with the break-up of Gondwana and from metasedimentary rocks of the Cordillera Darwin metamorphic complex, respectively. Exposures of these units occur in the hinterland of the Fuegian Andes thrust belt and inboard of the Patagonian–Fuegian batholith, suggesting that middle to late Eocene shortening in the rear of the Fuegian orogenic wedge structurally dammed batholith-derived sediment from reaching the foreland basin while contributing Jurassic and pre-Mesozoic detritus from hinterland thrust sheets. The timing of this interpreted deformation is in agreement with (1) independent structural, stratigraphic, and thermochronometric evidence of middle Paleogene deformation in the Fuegian Andes, and (2) marine Nd isotope ratio data that reveal initial penetration of Pacific water through the Drake Passage, thereby suggesting a possible link between the kinematics of the Fuegian Andes, the opening of Drake Passage, and if related, the Oi-1 glaciation of Antarctica.

Fluvial systems variations in the Rio Leona Formation: Tectonic and eustatic controls on the Oligocene evolution of the Austral (Magallanes) Basin, southernmost Argentina

The stratigraphy and sedimentology of the Oligocene Río Leona Formation (Austral/Magallanes Basin) is described and interpreted on the basis of a study of two sections located in the area close to Viedma and Argentino Lakes in southern Argentina. This nonmarine unit unconformably rests on top of the marine Eocene Man Aike Formation and is transitionally covered by the shallow marine, latest Oligocene–Early Miocene Centinela Formation. The Río Leona Formation is composed of varying percentages of conglomerates, sandstones, and mudstones, as well as thin but conspicuous levels of carbonaceous shales and volcaniclastic beds. The rocks represent sedimentation in different fluvial systems. Sedimentological data indicate a progressive change in the fluvial styles from high-energy braided systems to low-energy meandering and anastomosed systems. The landscape is envisaged to have evolved during the Oligocene from a high gradient piedmont area to a low-lying coastal plain. The Patagonian Sea flooded the area by the end of the Oligocene (22–25 My). Within the framework of the nonmarine sequence stratigraphic scheme for foreland basins, the Río Leona Formation deposits represent good examples of degradational, transitional, and aggradational systems tracks. The development of a basal unconformity and the erosion of the Eocene rocks suggest an initial uplift of the source area to the west. Changes in the fluvial systems indicate an increase of the accommodation space. The Oligocene evolution of this part of the Austral Basin probably reflects a combination of tectonic processes and eustatic sea-level changes. The former may be related to an episode of uplift in the southern Andes, followed by a period of tectonic quiescence and active volcanism with strong subsidence typical of foreland basin evolution. The latter may be linked to late Paleogene glaciation–deglaciation events in Antarctica.

New Fossil Platyrrhines from Santa Cruz Province, Argentina

New platyrrhine fossils have been recovered from early Miocene deposits near the Pinturas River in Santa Cruz Province, Argentina. Soriacebus ameghinorum is a saki-sized monkey with a deep V-shaped mandible, large procumbent incisors and small marmoset-like cheek teeth. Several other primate taxa are represented by dental, cranial, and post-cranial remains from the same deposits. These fossils document the presence of a phyletically and adaptively diverse fauna of primates in southernmost Argentina during the early Miocene.