Search for: All records

Abstract Drought has long been suspected as playing an important role in the abandonment of pre-Columbian Native American settlements across the midcontinental United States between 1350 and 1450 CE. However, high-resolution paleoclimatic reconstructions reflecting local effective moisture (the ratio of precipitation to evaporation) that are located in proximity to Mississippi period (1050–1450 CE) population centers are lacking. Here, we present a 1600-year-long decadally resolved oxygen isotope (δ¹⁸O) record from Horseshoe Lake (Collinsville, IL), an evaporatively influenced oxbow lake that is centrally located within the largest and mostly densely populated series of Mississippian settlements known as Greater Cahokia. A shift to higher δ¹⁸O in the Horseshoe Lake sediment record from 1200 to 1400 CE indicates that strongly evaporative conditions (i.e., low effective moisture) were persistent during the leadup to Cahokia’s abandonment. These results support the hypothesis that climate, and drought specifically, strongly impacted agriculturally based pre-Columbian Native American cultures in the midcontinental US and highlights the susceptibility of this region, presently a global food production center, to hydroclimate extremes. 

We compare hydrogen isotopic measurements of long-chain leaf-wax n-alkanes (2Hw; C27, C29, and C31) from Martin Lake, IN, United States of America (USA), with a calcite-based reconstruction of the oxygen isotopic composition of precipitation (18Op) from the same lake. We observe stable and high 2Hw during the Common Era (last 2000 years), which we interpret as growing-season precipitation originating mainly from the Gulf of Mexico and Atlantic. During the Little Ice Age (LIA; 1200-1850 CE), 2Hw values increased by 3-8 ‰, concomitant with a significant decrease in 18Op values by up to 12.5 ‰. Multiple proxy records for this time indicate persistent growing-season drought. We interpret these relatively high 2Hw values, as compared to the 18Op values, as a signal of low relative humidity that resulted in an 2H enrichment in plant source water resulting in high 2H values through enhanced plant water and/or soil evaporation. These results support the occurrence of low humidity conditions during the LIA in the midcontinental USA that also contributed to the marked decline of regional pre-Columbian Mississippian populations. 

Long-term relationships between mean-state climatic conditions and flood frequencies in the midcontinental United States (US) are not well established because instrumental records of fluvial processes are limited to the current warm period (CWP; the last ca. 150 years) and continuous paleo-flood records are exceedingly rare. Here, we investigate climate-flood relationships in the midcontinental US by reconstructing flood frequencies at Half Moon Pond, a 1600-year-old oxbow lake on the lower White River, Indiana (watershed ¼ ca, 29,000 km2). We used sediment accumulation rates and clastic fluxes constrained by high-resolution radiocarbon (14C) dating. Frequent flooding, as indicated by high sedimentation rates and clastic fluxes to Half Moon Pond, occurred leading up to and during the Medieval Climate Anomaly (MCA; 950e1250 CE) when paleoclimate records suggest the predominance of oceanatmosphere mean states resembling the negative phases of the Pacific Decadal Oscillation (-PDO-like) and Pacific North American Mode (-PNA-like). Reductions in sedimentation rates and clastic fluxes, indicating reduced flooding, subsequently occurred during the transition out of the MCA and into the Little Ice Age (LIA; 1250e1830 CE) as ocean-atmosphere conditions shifted to þ PDO-like and þPNA-like mean states. Sedimentation rates and clastic fluxes increased again after ca. 1800 CE, indicating increased flooding during the CWP as ocean-atmosphere conditions returned to -PDO-like and -PNA-like mean states. The White River trends were notably antiphased with sedimentation-rate-based flood frequencies for the lower Ohio River (500,000 km2 watershed) prior to 1830 CE. This antiphased relationship is consistent with flooding in moderate to small watersheds in the Midwest being sensitive to the occurrence of rainstorm events, which were more frequent leading up to and during the MCA, and flooding in large watersheds being more sensitive to large spring melts associated with extensive snowpacks, which characterized the LIA. That both the White and Ohio rivers experienced their most frequent flooding during the CWP suggests deforestation and changing land use practices increased flooding on Midwestern watersheds across scales despite a current climatic mean state that in the past only resulted in increased flooding on moderate to small watersheds. Continued increased in midcontinental rainfall are therefore likely to enhance the occurrence of floods in Midwestern watersheds across different geographic scales. 

Temperate broadleaf forests in eastern North America are diverse ecosystems whose vegetation composition has shifted over the last several millennia in response to climatic and human drivers. Yet, detailed records of long-term changes in vegetation composition and diversity in response to known periods of human activity, particularly multiple distinct periods of human activity at the same site, are still relatively sparse. In this study, we examine a sediment record from Avery Lake, Illinois, USA, using multiple metrics derived from pollen data to infer vegetation composition and diversity over the last 3,000 years. This 3,000-year history encompasses the Baumer (300 BCE–300 CE) and Mississippian settlements (1150–1450 CE) at Kincaid Mounds (adjacent to Avery Lake), and captures differences in the impact that these groups had on vegetation composition. Both groups actively cleared the local landscape for settlement and horticultural/agricultural purposes. Given the persistence of fire-tolerant Quercus in conjunction with declines in other tree taxa, this clearing likely occurred through the use of fire. We also apply a self-organized mapping technique to the multivariate pollen assemblages to identify similarities and differences in vegetation composition across time. Those results suggest that the vegetation surrounding Avery Lake was compositionally similar before and after the Baumer settlement, but compositionally different after the Mississippian settlement. The end of the Mississippian settlement occurred simultaneously with a regional shift in moisture characterized by drier summers and wetter winters associated with the Little Ice Age (1250–1850 CE), which likely prevented this ecosystem from returning to its pre-Mississippian composition.