Post by 1dave on Jul 30, 2018 2:41:06 GMT -5
Recent Geologic Mayhem
What happened 10-12,000 Years ago?
Disappearance of the mega fauna of North America, the pile of mammal bones in Bone Valley Florida . . .
www.earth-surf-dynam.net/6/271/2018/esurf-6-271-2018.pdf
Lots of charts and maps.
What happened 10-12,000 Years ago?
Disappearance of the mega fauna of North America, the pile of mammal bones in Bone Valley Florida . . .
www.earth-surf-dynam.net/6/271/2018/esurf-6-271-2018.pdf
Abstract.
The denudation history of active orogens is often interpreted in the context of modern climate gradients. Here we address the validity of this approach and ask what are the spatial and temporal variations in palaeoclimate for a latitudinally diverse range of active orogens?
We do this using high-resolution (T159, ca.80×80 km at the Equator) palaeoclimate simulations from the ECHAM5 global atmospheric general circulation model and a statistical cluster analysis of climate over different orogens (Andes, Himalayas, SE Alaska, Pacific NW USA). Time periods and boundary conditions considered include :
1. the Pliocene (PLIO, ∼3 Ma),
2. the Last Glacial Maximum (LGM, ∼ 21 ka),
3. mid-Holocene (MH, ∼ 6 ka), and
4. pre-industrial (PI, reference year 1850).
The regional simulated climates of each orogen are described by means of cluster analyses based on the variability in precipitation, 2 m air temperature, the intra-annual amplitude of these values, and monsoonal wind speeds where appropriate.
Results indicate the largest differences in the PI climate existed for the LGM and PLIO climates in the form of widespread cooling and reduced precipitation in the LGM and warming and enhanced precipitation during the PLIO.
The LGM climate shows the largest deviation in annual precipitation from the PI climate and shows enhanced precipitation in the temperate Andes and coastal regions for both SE Alaska and the US Pacific Northwest. Furthermore, LGM precipitation is reduced in the western Himalayas and enhanced in the eastern Himalayas, resulting in a shift of the wettest regional climates eastward along the orogen.
The cluster-analysis results also suggest more climatic variability across latitudes east of the Andes in the PLIO climate than in other time slice experiments conducted here.
Taken together, these results highlight significant changes in late Cenozoic regional climatology over the last ∼ 3 Myr. Comparison of simulated climate with proxy-based reconstructions for the MH and LGM reveal satisfactory to good performance of the model in reproducing precipitation changes, although in some cases discrepancies between neighbouring proxy observations highlight contradictions between proxy observations themselves.
Finally, we document regions where the largest magnitudes of late Cenozoic changes in precipitation and temperature occur and offer the highest potential for future observational studies that quantify the impact of climate change on denudation and weathering rates.
The denudation history of active orogens is often interpreted in the context of modern climate gradients. Here we address the validity of this approach and ask what are the spatial and temporal variations in palaeoclimate for a latitudinally diverse range of active orogens?
We do this using high-resolution (T159, ca.80×80 km at the Equator) palaeoclimate simulations from the ECHAM5 global atmospheric general circulation model and a statistical cluster analysis of climate over different orogens (Andes, Himalayas, SE Alaska, Pacific NW USA). Time periods and boundary conditions considered include :
1. the Pliocene (PLIO, ∼3 Ma),
2. the Last Glacial Maximum (LGM, ∼ 21 ka),
3. mid-Holocene (MH, ∼ 6 ka), and
4. pre-industrial (PI, reference year 1850).
The regional simulated climates of each orogen are described by means of cluster analyses based on the variability in precipitation, 2 m air temperature, the intra-annual amplitude of these values, and monsoonal wind speeds where appropriate.
Results indicate the largest differences in the PI climate existed for the LGM and PLIO climates in the form of widespread cooling and reduced precipitation in the LGM and warming and enhanced precipitation during the PLIO.
The LGM climate shows the largest deviation in annual precipitation from the PI climate and shows enhanced precipitation in the temperate Andes and coastal regions for both SE Alaska and the US Pacific Northwest. Furthermore, LGM precipitation is reduced in the western Himalayas and enhanced in the eastern Himalayas, resulting in a shift of the wettest regional climates eastward along the orogen.
The cluster-analysis results also suggest more climatic variability across latitudes east of the Andes in the PLIO climate than in other time slice experiments conducted here.
Taken together, these results highlight significant changes in late Cenozoic regional climatology over the last ∼ 3 Myr. Comparison of simulated climate with proxy-based reconstructions for the MH and LGM reveal satisfactory to good performance of the model in reproducing precipitation changes, although in some cases discrepancies between neighbouring proxy observations highlight contradictions between proxy observations themselves.
Finally, we document regions where the largest magnitudes of late Cenozoic changes in precipitation and temperature occur and offer the highest potential for future observational studies that quantify the impact of climate change on denudation and weathering rates.
Lots of charts and maps.
5 Conclusions
We present a statistical cluster-analysis-based description of the geographic coverage of possible distinct regional expressions of climates from four different time slices (Figs. 6, 9, 12, 15). These are determined with respect to a selection of variables that characterise the climate of the region and may be relevant to weathering and erosional processes. While the geographic distribution of climate remains similar throughout time (as indicated by results of four different climate states representative for the climate of the last 3 Myr), results for the PLIO simulation suggests more climatic variability east of the Andes (with respect to near-surface temperature, seasonal temperature amplitude, precipitation, seasonal precipitation amplitude and seasonal u-wind and v- wind speeds). Furthermore, the wetter climates in the South Asia region retreat eastward along the Himalayan orogen for the LGM simulation; this is due to decreased precipitation along the western part of the orogen and enhanced precipitation on the eastern end, possibly signifying more localised high erosion rates.
Most global trends of the high-resolution LGM and PLIO simulations conducted here are in general agreement with previous studies (Otto-Bliesner et al., 2006; Braconnot et al., 2007; Wei and Lohmann, 2012; Lohmann et al., 2013; R. Zhang et al., 2013, 2014; Stepanek and Lohmann, 2012).
The MH does not deviate notably from the PI, the LGM is relatively dry and cool, while the PLIO is comparably wet and warm. While the simulated regional changes in temperature and precipitation usually agree with the sign (or direction) of the simulated global changes, there are region-specific differences in the magnitude and direction. For example, the LGM precipitation of the tropical Andes does not deviate significantly from PI precipitation, whereas LGM precipitation in the temperate Andes is enhanced.
Comparisons to local, proxy-based reconstructions of MH and LGM precipitation in South Asia and South America reveal satisfactory performance of the model in simulating the reported differences. The model performs better for the LGM than the MH. We note however that compilations of proxy data such as we present here also identify inconsistences among neighbouring proxy data themselves, warranting caution in the extent to which both proxy data and palaeoclimate models are interpreted for MH climate change in South Asia and western South America.
The changes in regional climatology presented here are manifested, in part, by small to large magnitude changes in fluvial and hillslope relevant parameters such as precipitation and temperature. For the regions investigated here we find that precipitation differences among the PI, MH, LGM, and PLIO are in many areas around ± 200–600 mm yr−1, and locally can reach maximums of ±1000–2000 mm yr − 1 (Figs. 4, 7, 10, 13). In areas where significant precipitation increases are accompanied by changes in ice extent, such as parts of southern Alaska during the LGM, we would expect a shift in the erosional regime to glacier-dominated processes. Temperature differences between these same time periods are around 1–4◦C in many places, but reach maximum values of 8–10◦C.
Many of these maxima in the temperature differences geographically coincide with changes in ice sheet extent and must therefore be interpreted as part of a different erosional process domain. However, we also observe large temperature differences ( ∼ 5◦C) in unglaciated areas that would be affected by hillslope, frost cracking, and fluvial processes. The magnitude of these differences is not trivial, and will likely impact fluvial and hillslope erosion and sediment transport, as well as biotic and abiotic weathering. The regions of large-magnitude changes in precipitation and temperature documented here (Figs. 4, 7, 10, 13) offer the highest potential for future observational studies interested in quantifying the impact of climate change on denudation and weathering rates.
We present a statistical cluster-analysis-based description of the geographic coverage of possible distinct regional expressions of climates from four different time slices (Figs. 6, 9, 12, 15). These are determined with respect to a selection of variables that characterise the climate of the region and may be relevant to weathering and erosional processes. While the geographic distribution of climate remains similar throughout time (as indicated by results of four different climate states representative for the climate of the last 3 Myr), results for the PLIO simulation suggests more climatic variability east of the Andes (with respect to near-surface temperature, seasonal temperature amplitude, precipitation, seasonal precipitation amplitude and seasonal u-wind and v- wind speeds). Furthermore, the wetter climates in the South Asia region retreat eastward along the Himalayan orogen for the LGM simulation; this is due to decreased precipitation along the western part of the orogen and enhanced precipitation on the eastern end, possibly signifying more localised high erosion rates.
Most global trends of the high-resolution LGM and PLIO simulations conducted here are in general agreement with previous studies (Otto-Bliesner et al., 2006; Braconnot et al., 2007; Wei and Lohmann, 2012; Lohmann et al., 2013; R. Zhang et al., 2013, 2014; Stepanek and Lohmann, 2012).
The MH does not deviate notably from the PI, the LGM is relatively dry and cool, while the PLIO is comparably wet and warm. While the simulated regional changes in temperature and precipitation usually agree with the sign (or direction) of the simulated global changes, there are region-specific differences in the magnitude and direction. For example, the LGM precipitation of the tropical Andes does not deviate significantly from PI precipitation, whereas LGM precipitation in the temperate Andes is enhanced.
Comparisons to local, proxy-based reconstructions of MH and LGM precipitation in South Asia and South America reveal satisfactory performance of the model in simulating the reported differences. The model performs better for the LGM than the MH. We note however that compilations of proxy data such as we present here also identify inconsistences among neighbouring proxy data themselves, warranting caution in the extent to which both proxy data and palaeoclimate models are interpreted for MH climate change in South Asia and western South America.
The changes in regional climatology presented here are manifested, in part, by small to large magnitude changes in fluvial and hillslope relevant parameters such as precipitation and temperature. For the regions investigated here we find that precipitation differences among the PI, MH, LGM, and PLIO are in many areas around ± 200–600 mm yr−1, and locally can reach maximums of ±1000–2000 mm yr − 1 (Figs. 4, 7, 10, 13). In areas where significant precipitation increases are accompanied by changes in ice extent, such as parts of southern Alaska during the LGM, we would expect a shift in the erosional regime to glacier-dominated processes. Temperature differences between these same time periods are around 1–4◦C in many places, but reach maximum values of 8–10◦C.
Many of these maxima in the temperature differences geographically coincide with changes in ice sheet extent and must therefore be interpreted as part of a different erosional process domain. However, we also observe large temperature differences ( ∼ 5◦C) in unglaciated areas that would be affected by hillslope, frost cracking, and fluvial processes. The magnitude of these differences is not trivial, and will likely impact fluvial and hillslope erosion and sediment transport, as well as biotic and abiotic weathering. The regions of large-magnitude changes in precipitation and temperature documented here (Figs. 4, 7, 10, 13) offer the highest potential for future observational studies interested in quantifying the impact of climate change on denudation and weathering rates.
