Recent wildfire activity in semi-arid regions like western North America exceeds the range of historical records. High-resolution paleoclimate archives such as stalagmites could illuminate the link between hydroclimate, vegetation change, and fire activity in pre-anthropogenic climate states beyond the timescale of existing tree-ring records. Here we present an analysis of levoglucosan, a combustion-sensitive anhydrosugar, and lignin oxidation products (LOPs) in a stalagmite, reconstructing fire activity and vegetation composition in the California Coast Range across the 8.2 kyr event. Elevated levoglucosan concentrations suggest increased fire activity while altered LOP compositions indicate a shift toward more woody vegetation during the event. These changes are concurrent with increased hydroclimate volatility as shown by carbon and calcium isotope proxies. Together, these records suggest that climate whiplash (oscillations between extreme wetness and aridity) and fire activity in California, both projected to increase with anthropogenic climate change, were tightly coupled during the early Holocene.
C4 grassland ecosystems expanded across North America between ca. 8 and 3 Ma. Studies of ungulate enamel and environmental indicators from the middle Miocene Barstow Formation of southern California (USA) have demonstrated the presence of C4 vegetation prior to the late Miocene expansion of C4 grasslands. Fire promotes the growth of modern C4 grasslands and may have contributed to the Miocene expansion of C4 vegetation. We analyzed the concentration and accumulation rate (CHAR) of microscopic charred particles from sediment samples spanning the Barstow Formation in order to investigate the relationship between fire activity, canopy cover, and the presence of C4 vegetation. Concentration and CHAR were low throughout the formation then increased dramatically at 13.5 Ma. Enriched values of δ13C from soil organic matter and phytolith counts indicate the presence of C4 grasses and seasonally dry, open-canopy habitats at this time. The spike in concentration coincides with climatic cooling and drying in southern California after the Miocene Climatic Optimum. Increased fire activity may have contributed to habitat opening from forest to woodland and promoted the spread of C4 plants. This is the first charcoal record of fire activity from the middle Miocene of southwestern North America.
more » « less- Award ID(s):
- 1813996
- PAR ID:
- 10469920
- Publisher / Repository:
- Geological Society of America
- Date Published:
- Journal Name:
- Geology
- Volume:
- 51
- Issue:
- 8
- ISSN:
- 0091-7613
- Page Range / eLocation ID:
- 763 to 767
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract C4grasslands proliferated later in Australia than they did on other continents (∼3.5 Ma vs. 10–5 Ma). It remains unclear whether this delay reflects differences in climate conditions or ecological feedbacks, such as fire, that promote C4ecosystems. Here, we evaluated these factors using terrestrial biomarkers from marine sediments off western Australia. Fire‐derived polycyclic aromatic hydrocarbons (PAH) indicate fire ecology did not substantially change during or following C4expansion. The presence of fire‐adapted C3woody vegetation likely diminished the role of fire and delayed C4expansion until it was prompted by climate drying between 3.5 and 3.0 Ma. At the same time, mass accumulation rates of weathered PAHs increased 100‐fold, which indicates a significant loss of soil carbon accompanied this ecosystem shift. The tight couplings between hydroclimate and carbon storage altered boundary conditions for Australian ecosystems, and similar abrupt behavior may shape environmental responses to climate change.
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Abstract We reconstruct sea surface temperatures (SSTs) at Deep Sea Drilling Project Site 608 (42.836°N, 23.087°), north of the Azores Front, and Ocean Drilling Program Site 982 (57.516°N, 15.866°), under the North Atlantic Current, in order to track Miocene (23.1–5.3 Ma) development of North Atlantic surface waters. Mean annual SSTs from TEX86and UK′37proxy estimates at both sites were 10–15 °C higher than modern through the Miocene Climatic Optimum (17–14.5 Ma). During the global cooling of the Middle Miocene Climate Transition (~14.5–12.5 Ma), SSTs at midlatitude Site 608 cooled by ~6 °C, whereas high‐latitude Site 982 cooled by only ~2 °C, resulting in an ~4 Myr collapse of the SST gradient between the two sites. This regional pattern is inconsistent with an increased latitudinal surface temperature gradient, as generally associated with global cooling episodes linked to decreasing
p CO2levels. Instead, the pattern is best explained by enhanced ocean heat transport into the high‐latitude North Atlantic superimposed on the global cooling trend, probably due to enhanced Atlantic meridional overturning circulation and/or a stronger North Atlantic Current. During global late Miocene cooling (~8–7 Ma), surface waters cooled by ~6 °C at Site 982 while minimal change occurred at Site 608, reestablishing the North Atlantic SST gradient. The collapse and reemergence of the SST gradient between the middle‐ and high‐latitude North Atlantic suggests that interaction between changes in regional ocean circulation and the global response to changes in greenhouse gas concentration was important in Miocene climate evolution. -
Degree of canopy cover is linked to transpiration, carbon cycling and primary productivity of an ecosystem. In modern ecology, canopy structure is often quantified as Leaf Area Index (LAI), which is the amount of overstory leaf coverage relative to ground area. Although a key aspect of vegetation, the degree of canopy cover has proven difficult to reconstruct in deep time. One method, Reconstructed Leaf Area Index (rLAI), was developed to infer canopy structure using the relationship between non-grass leaf epidermal phytolith (plant biosilica) morphology, and leaf coverage in modern forests. This method leverages the observed correlation between epidermal phytolith size, shape (margin undulation), and light availability. When more light is available in a canopy, epidermal phytoliths tend to be smaller and less undulate, whereas less light availability is linked to larger and more undulate epidermal phytoliths. However, the calibration set used to develop this method was compiled from field sites and samples from localities in Costa Rica and it remains unclear how applicable it is to temperate North American fossil sites due to lack of data from relevant vegetation types and taxonomic differences between plant communities in the Neotropics vs. mid-latitude North America. For example, preliminary results measuring rLAI in phytolith assemblages from the Miocene of the North American Great Plains have yielded surprisingly high degrees of canopy density despite containing high relative abundances of open-habitat grasses. To test whether vegetational and taxonomic differences impact the calibration set, we constructed a new North American calibration using 24 quadrats from six sites, representing reasonable modern analogs for Miocene vegetation in eastern North America. Specifically, we sampled in Bennett Springs State Park in Lebanon, MO; Mark Twain National Forest in Rolla, MO; Tellico in Franklin, NC and Congaree National Park in Hopkins, SC. All sites include a range of canopy covers and vegetation types, from oak savannas and oak woodlands to mixed hardwood forests, pine savannas, and old growth bottomland forests. From each quadrat, we collected a soil sample and took hemispherical photos of the local canopy. From modern soil samples, biosilica was extracted in the lab, yielding phytolith assemblages which were scanned for epidermal phytoliths using a compound microscope. Recovered epidermal phytoliths size and margin undulation were measured and assemblage averages were used to predict measured LAI at each quadrat. Hemispherical photographs were processed using the software Gap Light Analyzer to obtain LAI values. We hypothesize there will be a linear relationship between actual LAI and LAI calculated from epidermal phytolith morphology, but its relationship will differ from that found in South America. Results will be used to reevaluate canopy coverage in sites within the Great Plains Miocene as well as applied to Pacific Northwest Miocene sites, both to understand changes to vegetation during global climatic events in their respective regions.more » « less
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The fossil record of the U.S. Pacific Northwest preserves many Middle Miocene floras with potential for revealing long-term climate-vegetation dynamics during the Miocene Climatic Optimum. However, the possibility of strong, eccentricity-paced climate oscillations and concurrent, intense volcanism may obscure the signature of prevailing, long-term Miocene climate change. To test the hypothesis that volcanic disturbance drove Middle Miocene vegetation dynamics, high-resolution, stratigraphic pollen records and other paleobotanical data from nine localities of the Sucker Creek Formation were combined with sedimentological and geochemical evidence of disturbance within an updated chronostratigraphic framework based on new U-Pb zircon ages from tuffs. The new ages establish a refined, minimum temporal extent of the Sucker Creek Formation, ~15.8 to ~14.8 Ma, and greatly revise the local and regional chronostratigraphic correlations of its dispersed outcrop belt. Our paleoecological analysis at one ~15.52 Ma locality reveals two abrupt shifts in pollen spectra coinciding with the deposition of thick ash-flow tuffs, wherein vegetation dominated by Cupressaceae/Taxaceae, probably representing a
Glyptostrobus oregonensis swamp, and upland conifers was supplanted by early-successional forests with abundantAlnus andBetula . Another ephemeral shift from Cupressaceae/Taxaceae swamp taxa in favor of upland conifersPinus andTsuga correlates with a shift from low-Ti shale to high-Ti claystone, suggesting a link between altered surface hydrology and vegetation. In total, three rapid vegetation shifts coincide with ash-flow tuffs and are attributed to volcanic disturbance. Longer-term variability between localities, spanning ~1 Myr of the Miocene Climatic Optimum, is chiefly attributed to eccentricity-paced climate change. Overall, Succor Creek plant associations changed frequently over ≤105years timespans, reminiscent of Quaternary vegetation records. Succor Creek stratigraphic palynology suggests that numerous and extensive collection of stratigraphically controlled samples is necessary to understand broader vegetation trends through time.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1130/G50881.1
