Ben Hardt

A high-resolution stalagmite record of the Holocene East Asian monsoon from Mt Shennongjia, central China

High-resolution oxygen isotope (δ18O) profiles of six stalagmites from Sanbao Cave in Hubei province, central China, established with 1413 oxygen isotope data and 65 230Th ages, provide a continuous history of East Asian Summer Monsoon (EASM) intensity for the period from 13—0.2 thousand years before present (ky BP, relative to AD 1950). The δ 18O record includes four distinct stages in the evolution of the EASM: (1) an abrupt transition (~11.5 ky BP) into the Holocene; (2) a period of gradual increase in monsoon intensity (11.5—9.5 ky BP); (3) the maximum humid period (9.5—6.5 ky BP); and (4) a period of gradual decline in monsoon intensity (6.5—0.2 ky BP). Comparison of Sanbao with regional records of comparable resolution reveals that the timing of the beginning and end of the Holocene Optimum (as defined by the minimum in δ18 O) was similar in the Indian and East Asian monsoon systems. This supports the idea that shifts in the monsoon tied to shifts in the mean position of the Inter-Tropical Convergence Zone (ITCZ) may control monsoon intensity throughout the entire low-latitude region of Asia on orbital timescales. This observation also supports the idea that the fluctuations in δ18 O recorded across southern Asia reflect broad changes in the monsoon, as opposed to local meteoric precipitation. The EASM records from Sanbao largely follow orbital-scale insolation changes, yet exhibit similar variability to Greenland ice core δ18O on millennial to centennial scales during the early to middle Holocene (r = 0.94).

Multiproxy evidence from caves of Native Americans altering the overlying landscape during the late Holocene of east-central North America

We compare environmental changes recorded in stalagmites and alluvium from the mountainous Buckeye Creek basin of West Virginia, USA to a nearby, independent archaeological record of Native American presences in the forested watershed. A climatic record constructed from stable isotopic (δ18O calc and δ13Ccalc) and trace metal (Sr/Ca) ratios in stalagmitic calcite is consistent with regional palynology during much of the Holocene. The stalagmite δ13Ccalc and Sr/Ca values track aridity associated with North Atlantic Ocean (NAO) ice-rafting events during solar minima. However, the δ13Ccalc record diverges sharply from the Sr/Ca record at ~2100 (calendar) years BP, which maintains the same relationship with ice rafting in the NAO. A dramatic and sustained enrichment in δ13Ccalc values (>1‰) without a corresponding shift in Sr/Ca values, suggests a systemic change in above-cave vegetation and soil carbon. This hypothesis is corroborated by a record of the stable isotopic composition of bulk organic carbon (δ 13Corg) in alluvial silts. Cultural artefacts record Native American presences in the watershed during the late Holocene and archaeologists place peak Native American presence as having occurred between 750 and 550 years BP, nearly contemporaneous with peaks in δ13Ccalc , δ13Corg, and relative charcoal abundances documented herein. Notably, values of the three environmental proxies decrease after Native Americans abandoned the watershed. The available evidence is consistent with Native Americans having made significant changes to the area’s ecosystem and soils prior to the arrival of Euro-colonial peoples at ~225 years BP. Our findings highlight the active roles native peoples had in shaping the North American “wilderness” described prior to its destruction by early European settlers.

Long-term changes in precipitation recorded by magnetic minerals in speleothems

Speleothems are important paleoclimate archives. Researchers typically compile measurements of stable isotopic ratios dated using high precision U-Th radiometric techniques to reconstruct regional and global climate. Magnetic material incorporated within speleothems can provide an independent means of connecting large-scale climatic changes with their impact on more localized processes in soils overlying cave systems. Under certain environmental conditions, pedogenic processes can produce magnetite nanoparticles. Enhancement of pedogenic magnetite in soil profiles depends strongly on local precipitation. Pedogenic magnetite can be subsequently transferred via drip-waters into underlying cave-systems and incorporated into speleothems as they grow. Here, we employ high-resolution magnetic methods to analyze a well-dated stalagmite from Buckeye Creek Cave, West Virginia (USA), and find that changes in magnetite concentration follow both changes in stable isotopes measured in the same stalagmite and global climate proxies. We interpret the changes in magnetite concentration as reflecting variations in local pedogenic processes, controlled by changes in regional precipitation. This record demonstrates how magnetic measurements on speleothems can constrain interpretations of speleothem climate proxies.

Stalagmite multi-proxy evidence of wet and dry intervals in northeastern Namibia: Linkage to latitudinal shifts of the Inter-Tropical Convergence Zone and changing solar activity from AD 1400 to 1950

Multiple proxies using variation in δ18O, δ13C, mineralogy, and petrography in a newly generated high-resolution record of Stalagmite DP1 from Dante Cave indicate a linkage between changes in hydroclimate in northeastern Namibia and changes in solar activity and changes in global temperatures. The record suggests that during solar minima and globally cooler conditions (ca. 1660–1710 and ca. 1790–1830), wetter periods (reflecting longer summer seasons) in northeastern Namibia were linked to advances of the Inter-Tropical Convergence Zone (ITCZ) and the Inter-Ocean Convergence Zone (IOCZ) southwestward. A slight southward push of the Angola–Benguela Front (ABF) during such intervals could also be expected, bringing more rainfall inland. On the other hand, drier and warmer periods in northeastern Namibia, inferred from the increasing δ18O trend in Stalagmite DP1 after AD 1715, coincide with globally warmer conditions, and thus a northeastward migration of the ITCZ, specifically with more warming of the Northern Hemisphere (NH). This finding agrees with reducing precipitation observed in the summer rainfall zone of southern Africa since ca. 1900. Therefore, predictions of warming in high-latitude regions of the NH in the next century should suggest that the presently semi-arid climate of northern Namibia may become even drier.

East central North America climates during marine isotope stages 3–5

Long-term, high-resolution stalagmite carbon and oxygen isotope records from eastern North America (ENA) provide a midlatitude history of relative changes in moisture availability and climate states during the last interglacial and glacial inception (127.7 to 41.6 ka before present). The West Virginia carbon record shows low-amplitude variability at orbital time scales, superimposed on a long-term asymmetric pattern similar to global sea level changes. Relative moisture availability peaked at ~114 ka, and following a brief dry interval at ~96 ka, moisture availability gradually decreased. The almost linear change in moisture availability over ENA may reflect gradual changes in midlatitude zonal circulation as the polar cell and Laurentide Ice Sheet expanded or decreased. In contrast, our oxygen record is precession modulated and in phase with spring insolation, perhaps due to changes in precipitation seasonality. The separate pacings by eccentricity (carbon) and precession (oxygen) expose an underlying complexity that will be a challenge to explain.

Groundwater lowering and stream incision rates in the Central Appalachian Mountains of West Virginia, USA

Surface channel incision rates are of broad geomorphological interest because they set the boundary conditions for landscape change by affecting changes in local relief and hillslope angles. We report groundwater table lowering rates associated with subsurface Buckeye Creek and the surface channel of Spring Creek in southeastern West Virginia, USA. The mountainous watersheds have drainage areas of 14 km 2 and 171 km 2 , respectively. The lowering rates are derived from U/Th-dating of stalagmites and the paleomagnetostratigraphy of clastic sediments in Buckeye Creek Cave. The oldest stalagmites have a minimum age of 0.54 Ma and we use a minimum age of 0.778 Ma for clastic cave sediments deposited during a period of reversed magnetic polarity. The water table at Buckeye Creek has lowered at a rate of ≤40 m Ma -1 . Based on the relative elevations of Buckeye and Spring creeks, the water table at Spring Creek has lowered at a rate of ≤47 m Ma -1 . These values are consistent with previously published rates obtained from caves in the region, although those rates were reported as surface channel incision rates, based on the assumption local groundwaters drained to the surface channel of interest. However, the rates we report are almost certainly not simple bedrock incision rates because of karst processes acting within the cave and surrounding, well-developed fluviokarst (e.g., stream capture). Caveats aside, incision rates of ≤47 m Ma -1 now appear typical of landscapes of the Appalachian Mountains and Plateau.