B44C-01 16:00h
Comparison of Holocene fire-history records from temperate latitudes of western North and South America
During the last 20 kyr, fire-history records from the western U.S. and Patagonia display similar patterns and timing despite differences in the large-scale controls of climate between the two hemispheres. In summer-dry areas of northwestern North America, charcoal levels rise after 12 ka and the distribution of charcoal peaks indicates higher-than-present fire frequency until ca. 5-7 ka. High fire occurrence is attributed to environmental changes arising from the amplification of the seasonal cycle of insolation in the early Holocene, including the enhancement of the subtropical high-pressure system and increased summer drought. In Patagonia, charcoal records from lat 40-46S, and south of lat 50S also registered high fire activity between 12 and 6-8 ka, a time when summer insolation was less than at present but winter insolation was greater than at present. High fire occurrence may be explained by the location of winter and summer storm tracks between lat 46 and 50S, an earlier fire season due to warmer winters, and a pattern of interannual variability that favored recurring drought in the early Holocene. Comparable to the Rio Rubens record at lat 52S (Huber et al. 2004), L. Pollux, Chile (45.6917S, 71.6917W) records high fire frequency associated with parkland vegetation. L. Trebol (Lat. 41.07125S, Long. 79.4161W) farther north also features frequent fires during a parkland period and later with the expansion of {\it Nothofagus-Austrocedrus} woodland. In all three sites, the fire frequency declined with the development of closed {\it Nothofagus} forest in the late Holocene. The synchroneity in fire activity patterns in the two hemispheres is consistent with that observed in other terrestrial paleoclimate records from the mid- and high latitudes. It suggests that the proximal controls of fire occurrence on millennial time scales are ultimately driven by insolation and concomitant changes in Pacific Ocean- mediated interannual variability.
B44C-02 16:15h
Synchroneity in Background Charcoal Records From the Northwestern U.S.
Holocene fire-histories have been reconstructed from charcoal accumulation rates (CHAR) in lakes, bogs, and forest soils. "Peaks" in the CHAR data represent discrete fire events or intervals with frequent fires, while slowly varying trends represent "background" changes in regional burning, vegetation (biomass), and/or depositional processes. Background CHAR (BCHAR) data were explored in 15 high-resolution macroscopic charcoal records from the northwestern U.S. to identify widespread patterns. BCHAR was compared with charcoal peak influx and pollen data from the same sediment cores and with geospatial data to explain the patterns. Relative charcoal abundances at different sites were largely explained by local characteristics including lake size and elevation, and watershed area and mean slope. Synchroneity in gradual BCHAR trends, abrupt changes, and variability is further evidence that background variations are controlled primarily by broad-scale forcings, such as changing climate conditions or human activity. During the Holocene, BCHAR rose steadily and reached a maximum at ca. 1600 cal yr BP (AD 400), after which values generally declined to a late-Holocene minimum during the Little Ice Age (600-100 cal yr BP). Background charcoal increased again in the last few centuries. Fire frequency inferred from CHAR peaks showed different long-term trends than the BCHAR data, however fires were very frequent at several sites at ca. 2000 cal yr BP, when BCHAR was near its maximum. Comparisons between BCHAR and arboreal pollen percentages showed that fire activity and fuel levels were highly correlated on centennial and millennial time-scales. These results complement our understanding of northwestern U.S. fire-history based on local fire frequency data. They also demonstrate the sensitivity of high-resolution macroscopic charcoal records to long-term vegetation and climate dynamics.
B44C-03 16:30h
High temporal Resolution Fire History in Eastern Africa: the Last 16 kyr cal. BP
Charcoal series, based on a high temporal resolution analysis, at Lake Massoko (9°20' S, 33° 45' E, 770 a.s.l., SW Tanzania) reveals numerous changes of fire regime over the last 16 kyr cal BP. Data are based on the tallying of charcoal from 700 continuous 1 cm thick samples along a 7m long sequence. The structure of charcoal particles is well preserved with an length:width ratio superior to 5; this appears to testify to the local provenance of the material studied and to the rapid transport of particles to the lake. The majority of particles belongs to herbaceous cuticles produced by savanna or bush fires. Time control is supported by 14 radiocarbon dates. Mean time resolution per sample of 17 yr provides the first long detailed biomass burning record in Africa. This record evidences frequent fires events during the last 16 kyr, indicating that fire is a key component of east African ecosystems since, at least, the last glacial stage up to present. From 16 to 12 kyr cal BP, the charcoal influx into the lake is low except during two specific sequences, between 13.5-12.5 kyr cal BP and 14.5-14 kyr cal BP. These may correspond to more arid or more biomass-available phases. Around 10 kyr cal BP, at the early Holocene a greater influx of charcoal is recorded in the lake, probably as a result of a high fire regime likely triggered by severe droughts. Between 8.3 and 1.7 kyr cal BP, the charcoal influx displays a cyclic fire history of ca. 500 yr. Low fire regime, between 3.5-2.5, 5.5-5 and 7.5-7 kyr cal BP, correspond to wetter periods. About 12 sequences of fire increase and decrease are highlighted, which appears to support a high climatic variability during the middle-Holocene. After 1.7 kyr BP, there is a long lasting increase of charcoal influx into the lake, as observed by black carbon analysis (Thevenon et al., 2003). This particular period, without analog since 16 kyr cal BP, is consistent with the development of Iron Age settlements in the region, slash-and-burn agriculture and animal husbandry fire practices. This work was supported by the French CNRS-INSU-FNS, ECLIPSE (CLEHA) and RESOLVE programs.
B44C-04 16:45h
Holocene Fires and Atmospheric Carbon Emissions Modeling in Eastern Canada
We modeled the atmospheric carbon released by paleofires from Quebec and Ontario, eastern Canada. The terms of the model include the biome areas, the net rate of carbon released per biome from biomass burning and the fire anomaly per ka versus present-day (0ka) deduced from charcoal series of 37 lakes. Over the Postglacial, the Taiga zone does not matches the pattern of fire history and carbon released of Boreal, Boreal Atlantic Maritime and Mixed Wood Plains zones because of different air masses influences. Our focus on 6ka and 3ka shows that the role of the Mixed Wood Plains and the Boreal Atlantic Maritime zones on the total carbon emissions by fires is negligible both at 6 and 3ka. At 6ka, the Taiga zone plays a key role, while at 3ka the Taiga and the Boreal zones display equivalent contributions to the total carbon released to the atmosphere. The role of fires at 6ka and at 0ka is similar on the total atmospheric carbon mass emission despite changes in biomass burning activity. The role of Taiga at 6ka is compensated by Boreal zone at 0ka. However, the carbon emission at 3ka is significantly higher (ca 30%) than at 6 and 0ka because of sustained high fire activity both in Taiga and Boreal zones. Long-term climatic changes affect the biomass burning activity that acts together with vegetation cover on the global carbon cycle.
B44C-05 INVITED 17:00h
Scale-Dependent Controls of Late-Holocene Forest Fires in British Columbia: Insights From Intra-regional Paleorecord Comparisons
Forest fire regimes are affected by multiple controls that operate at local (e.g. stochastic ignitions, topography, and fuel loads) to regional (e.g. climate) scales. At small spatial scales, climatic controls may be obfuscated by local factors, but the spatial scale at which climatic controls outweigh local controls is poorly understood for stand-replacing fire regimes. We addressed this hierarchy of controls by comparing Holocene fire histories (the specific times of fire events and trends in fire frequency) and fire regimes (the statistical distribution of fire intervals) based on sediment charcoal records from southern British Columbia. We compared two pairs of charcoal records. First, at sites with similar vegetation and climatic history, we expected similar fire regimes and histories. However, a comparison of two charcoal records from sites in similar vegetation only 11-km apart revealed different fire-interval distributions and the lack of any synchrony in fire dates $<$ 2500 years before present (BP). After 2500 years BP, the fire regimes converged and fire events were marginally synchronous. In addition, the composite fire-frequency record (average of both sites) showed strong coherency with regional climatic changes evidenced by late-Holocene glacial advances. This coherency probably resulted from the increased area represented by the composite record (up to ~200 ha) and from the increased regional climatic variability over the last several millennia. Secondly, at sites in different elevational forest zones, we expected different fire regimes and possibly different fire histories. In this comparison, charcoal accumulation rates suggested that prior to 3000 years BP, fire severity was high at the high elevation site (1530 m asl) and low at the low elevation site (515 m asl) site, such that charcoal peaks could not be identified at the low elevation site. After 3000 years BP, severity increased at the low elevation site, allowing distinct charcoal peaks to be identified. During this period, statistically significant synchrony was detected within 1000-year and ca. 300-year windows, suggesting climatic variability at these scales entrained fire history over the last few millennia. Overall, these results indicate 1) that stands with similar modern conditions may have experienced different past fire regimes and fire history, likely because local processes outweighed the synchronizing effect of climate over millennia, and 2) that the influence of climate varied over time, and was stronger during the past 2500 years than before because of greater climatic variability.
B44C-06 17:15h
Fire, Holocene Climate Change, and Geomorphic Response Recorded in Alluvial Fan Sediments
Alluvial fan stratigraphic sequences record fire history in charcoal-rich deposits and buried burned soil surfaces. Deposit characteristics provide information about the magnitude of fire-related sedimentation events and severity of associated fires, and radiocarbon-dating of charcoal establishes the timing of fires. Unlike lakes, alluvial fans are ubiquitous in mountain environments. Although alluvial-fan fire records lack the annual resolution of tree-ring records, compilation of data from many alluvial fan sites provides a statistical sample of fire timing and approximate severity that can be related to climate variations over centennial to millennial timescales. We examine alluvial fan records from xeric Pinus ponderosa-dominated forests of central Idaho, and compare them with similar records from cooler, high-elevation Pinus contorta-dominated forests of Yellowstone National Park. Identification of charcoal macrofossils from Idaho fan deposits limits inbuilt age errors in radiocarbon dating, and shows that similar forest compositions have existed over the last ca. 4000 years in the fan drainage basins. Limited data from ca. 4000-7000 yr ago suggest that Pinus ponderosa was either sparse or absent in the 4 basins represented. Large fire-related debris flows in both Idaho and Yellowstone indicate severe fires during the ca. 1050-750 cal yr BP Medieval Climatic Anomaly (MCA), which included widespread and severe western US droughts. Another such episode 2700-1600 cal yr BP is less prominent in the Idaho record. Numerous small, fire-induced sedimentation events in Idaho ca. 350-500 (Little Ice Age), 1200-1400, and 2800-3000 cal yr BP likely indicate frequent low- to mixed-severity fires, and coincide with indicators of generally cool, moist conditions in the western USA and North Atlantic, and with minimal fire activity in Yellowstone. We infer that these effectively wetter periods allow greater grass growth, fueling frequent surface fires in ponderosa forests, but limiting fires in general in the effectively wetter forests of Yellowstone. Maxima in dated small events may relate to significant droughts within these intervals, e.g. in the late AD 1500s, but widespread severe fires are not indicated. Alluvial-fan records add to data from other charcoal-based proxy records of fire that indicate the importance of centennial- to millennial-scale climate change in modulating fire activity and geomorphic response in conifer forests over the Holocene.
B44C-07 17:30h
Holocene Charcoal Deposition From Brazilian Forest Fires
Determination of charcoal accumulation rate in lacustrine sediments allows to reconstruct the fire history of the region surrounding the lake. Our studies have been achieved in three Amazonian sites and one site in Atlantic rainforest. Charcoal fragments are identified and counted under a microscope. Typical size of these charcoals is around ten micrometers and they probably have been subject to eolian transport. The highest charcoal accumulation rates were obtained in sediments from Middle Holocene in Carajas region, eastern Amazonia. These rates are on the same order than the present day charcoal accumulation rate in Alta Floresta, a region of Amazonia which is being submited to intense slash and burn. The lowest values were found in Lagoa da Pata in Sao Gabriel da Cachoeira, a very humid area in western Amazon. We observed from the D. Helvecio record, in the Atlantic rainforest, fire occurrences from 8,400 to 6,400 cal years BP. For Carajas lake, surrounded by tropical rain forest, we had identified fires during the period between 8,000 and 5,300 cal years BP. Finally, the lake Caracarana, which is surrounded by grass savanna, showed a record of main fire occurrence phase at 9,750 cal yrs BP and a second phase marked by charcoal peaks at 7,680, 6,990 and 6,460 cal yrs BP. The synchronism of the fire occurrence periods in different Brazilian regions is related to the Middle Holocene dry climate phase provoked by the low summer insolation. Differences in the accumulation rates can be attributed to differences in biomass availability and fire return time. The carbon released in the atmosphere by this fires must have contributed to the observed increase of CO2, poorer in 13C, during the middle Holocene.
B44C-08 17:45h
Assessing the Evidence for Extensive Wildfires at the Cretaceous-Tertiary Boundary
Models of the Cretaceous-Tertiary impact at Chicxulub have suggested that the thermal radiation released by the impact would have been sufficient to ignite extensive wildfires. Eight non-marine K-T sequences stretching from New Mexico to Saskatchewan have been studied in order to test this hypothesis. A multi-proxy approach has been devised by identifying and using key palaeo wildfire proxies (charcoal, soot and polyaromatic hydrocarbons (PAH's)) in combination to assess the extent of biomass burning as part of the K-T events. Soot and PAH's cannot be used to indicate fire location, as soot and PAH's from one large fire could be spread globally. The morphology of the soot and nature of the PAH's present can be used to determine their source, allowing identification of those created by biomass burning versus those from coal, gas and hydrocarbons in the K-T rocks. In contrast to soot and PAH's charcoal is a product uniquely produced by the combustion of vegetation. Charcoal in non-marine rocks provides an excellent tool to record the distribution of wildfires and therefore assess the extent of any thermal radiation associated with the impact at Chicxulub. Quantitative data from three different measures of charcoal abundance ({\it in situ} in polished blocks of rock and macro- and microscopic charcoal particles released from sieving of demineralised sediment) reveal that the K-T boundary rocks across the Western Interior of North America contain significantly less charcoal than is typical of the Cretaceous background of this area. The Cretaceous sedimentary rocks contain between 4 and 9 times (according to the measure used) more charcoal particles than the K-T sedimentary rocks. Taphonomic factors do not explain this difference. Furthermore non-charred plant remains are also abundant in the K-T rock layers. Re-assessment of the record of soot and PAH's reported in the K-T rocks suggests that the morphology of the soot and the signature of the PAH's is more consistent with them being sourced from the vaporization of hydrocarbon material rather than biomass burning. We conclude that there was no significant wildfire across North America as part of the K-T events. The below background levels of charcoal in the K-T rocks allows the ground temperatures following the K-T impact to be constrained to no more than $545\deg$C at any point and not above $325\deg$C for any significant period. This indicates that the impact at Chicxulub did not generate sufficient thermal power to ignite extensive wildfires.