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null (Ed.)Abstract Carboniferous–Permian strata in basins within the Central Pangean Mountains in France archive regional paleoequatorial climate during a unique interval in geological history (Pangea assembly, ice-age collapse, megamonsoon inception). The voluminous (∼1.5 km) succession of exclusively fine-grained red beds that comprises the Permian Salagou Formation (Lodève Basin, France) has long been interpreted to record either lacustrine or fluvial deposition, primarily based on a local emphasis of subaqueous features in the upper ∼25% of the section. In contrast, data presented here indicate that the lower-middle Salagou Formation is dominated by up to 15-m-thick beds of internally massive red mudstone with abundant pedogenic features (microscale) and no evidence of channeling. Up-section, limited occurrences of ripple and hummocky cross-stratification, and mudcracks record the intermittent influence of shallow water, but with no channeling nor units with grain sizes exceeding coarse silt. These data suggest that the most parsimonious interpretation for the Salagou Formation involves eolian transport of the sediment and ultimate deposition as loess in shallow, ephemeral lacustrine environments. Provenance analyses of the Salagou Formation indicate coarse-grained protoliths and, together with geochemical proxies (chemical index of alteration [CIA] and τNa) that correspond respectively to a low degree of chemical weathering and a mean annual temperature of ∼4 °C, suggest that silt generation in this case is most consistent with cold-weathering (glacial and associated periglacial) processes in the Variscan highlands. Together with previous studies that detailed voluminous Permian loess in western equatorial Pangea, this work shows a globally unique distribution of dust at low latitudes that can be linked either directly to glaciated alpine terranes or to reworked and deflated deposits of other types (e.g., fluvial outwash) where fine-grained material was originally generated from glacial grinding in alpine systems. These results further support a revised model for early Permian climate, in which extratropical ice sheets coexisted with a semiarid tropics that may have hosted significant ice at moderate elevation.more » « less
Atmospheric rivers (ARs) are an important driver of surface mass balance over today's Greenland and Antarctic ice sheets. Using paleoclimate simulations with the Community Earth System Model, we find ARs also had a key influence on the extensive ice sheets of the Last Glacial Maximum (LGM). ARs provide up to 53% of total precipitation along the margins of the eastern Laurentide ice sheet and up to 22%–27% of precipitation along the margins of the Patagonian, western Cordilleran, and western Fennoscandian ice sheets. Despite overall cold conditions at the LGM, surface temperatures during AR events are often above freezing, resulting in more rain than snow along ice sheet margins and conditions that promote surface melt. The results suggest ARs may have had an important role in ice sheet growth and melt during previous glacial periods and may have accelerated ice sheet retreat following the LGM.
Oneida Lake, New York, is the remnant of Glacial Lake Iroquois, a large proglacial lake that delivered fresh water to the Atlantic Ocean during the last deglaciation. The formation of Glacial Lake Iroquois and its subsequent drainage into the Atlantic Ocean via the Mohawk Valley was a significant shift in the routing of Laurentide Ice Sheet meltwater to the east instead of south via the Allegheny or Susquehanna Rivers. Catastrophic drainage of Glacial Lake Iroquois into the Atlantic Ocean via the Champlain Valley is interpreted as the meltwater pulse responsible for the Intra-Allerod cold stadial. Therefore, understanding the evolution of Glacial Lake Iroquois has significant implications for understanding late Pleistocene paleoclimate. High-resolution CHIRP seismic reflection data provides insight into the evolution of Glacial Lake Iroquois and Oneida Lake. Three seismic units image distinct stages of the Oneida Basin. Unit 1 is interpreted as proglacial lake deposits that overlie glacial till. Unit 2 is interpreted as sediments deposited when the Oneida Basin became isolated from Glacial Lake Iroquois and Unit 3 is interpreted as lacustrine sediments of the modern lake. Distally sourced turbidites possibly triggered by seismic activity or ice sheet meltwater pulses are represented as reflection- free acoustic facies that infill topographic lows and range in thickness from ~1–5m within otherwise conformable proglacial lake deposits. Local slump deposits imaged at the boundary between Unit 1 and 2 were likely triggered by the drainage of Glacial Lake Iroquois. Wave cut terraces indicative of a low stand on the upper bounding surface of Unit 2 are likely the result of drier conditions during the Holocene Hypsithermal. Furthermore, preservation of this low stand suggests a rapid rise in lake level, possibly the result of the same transition to a wetter climate responsible for the Nipissing transgression observed in the Laurentian Great lakes.more » « less
Permocarboniferous strata of basins proximal to the Central Pangaean Mountains in France archive regional paleoequatorial climate during a unique interval in geological history (late Paleozoic Pangaean assembly, ice age collapse, megamonsoon inception). The voluminous (estimated 2 km) succession of exclusively fine-grained redbeds that composes the Permian Salagou Formation (Lodéve Basin, France) has been interpreted as recording either lacustrine or fluvial settings. We present preliminary field data to explore the hypothesis that these deposits record eolian transport, and ultimate deposition as either loess or in a shallow lacustrine environment. Fieldwork includes ~1000 m of section described at dm-scale, and magnetic susceptibility measured at 0.5 m intervals, from sections strategically located in both proximal and distal areas, and from all stratigraphic levels of the unit to assess spatial and temporal variations. These data indicate that the lower and middle Salagou Formation is dominated by internally massive, red mud-siltstone with no evidence of channeling. Up-section, a higher frequency of ripples, rare hummocky cross stratification, and mudcracks record the presence of shallow water, but with no channeling, nor units of grain size exceeding very fine-grained sand. Randomly-oriented slickensides at various localities in the mid-upper Salagou may represent incipient pedogenesis. The lack of evidence for channels and other fluvial features casts doubt on a fluvial interpretation. A lacustrine interpretation is consistent with local evidence of shallow water. However, in the absence of fluvial transport indicators, large volumes of entirely fine-grained material that were delivered to the Lodéve basin call for eolian transport, and thus a loess or shallow lacustrine interpretation. The documentation of voluminous paleoloess in eastern equatorial Pangea during the Permian could reflect the influence of glaciation associated with the Variscan highlands. Together with previous studies that detail Permian loess in western equatorial Pangea, this work impacts our understandingof the global Late Paleozoic climate system and presents a need to reevaluate modeling parameters (e.g. equatorial mountain glaciation, atmospheric dust loading).more » « less
Bulk sediment δ15N records from the eastern tropical Pacific (ETP) extending back to the last ice age most often show low glacial δ15N, then a deglacial δ15N maximum, followed by a gradual decline to a late Holocene δ15N that is typically higher than that of the Last Glacial Maximum (LGM). The lower δ15N of the LGM has been interpreted to reflect an ice age reduction in water column denitrification. We report foraminifera shell‐bound nitrogen isotope (FB‐δ15N) measurements for the two species
Neogloboquadrina dutertreiand Neogloboquadrina incomptaover the last 35 ka in two sediment cores from the eastern equatorial Pacific (EEP), both of which have the typical LGM‐to‐Holocene increase in bulk sediment δ15N. FB‐δ15N contrasts with bulk sediment δ15N by not indicating a lower δ15N during the LGM. Instead, the FB‐δ15N records are dominated by a deglacial δ15N maximum, with comparable LGM and Holocene values. The lower LGM δ15N of the bulk sediment records may be an artifact, possibly related to greater exogenous N inputs and/or weaker sedimentary diagenesis during the LGM. The new data raise the possibility that the previously inferred glacial reduction in ETP water column denitrification was incorrect. A review of reconstructed ice age conditions and geochemical box model output provides mechanistic support for this possibility. However, equatorial ocean circulation and nitrate‐rich surface water overlying both core sites allow for other possible interpretations, calling for replication at non‐equatorial ETP sites.