Lipid Biomarker Record Documents Hydroclimatic Variability of the Mississippi River Basin During the Common Era

Abstract

Floods and droughts in the Mississippi River basin are perennial hazards that cause severe economic disruption. Here we develop and analyze a new lipid biomarker record from Horseshoe Lake (Illinois, USA) to evaluate the climatic conditions associated with hydroclimatic extremes that occurred in this region over the last 1,800 years. We present geochemical proxy evidence of temperature and moisture variability using branched glycerol dialkyl glycerol tetraethers (brGDGTs) and plant leaf wax hydrogen isotopic composition (δ 2 H wax) and use isotope-enabled coupled model simulations to diagnose the controls on these proxies. Our data show pronounced warming during the Medieval era (CE 1000-1,600) that corresponds to midcontinental megadroughts. Severe floods on the upper Mississippi River basin also occurred during the Medieval era and correspond to periods of enhanced warm-season moisture. Our findings imply that projected increases in temperature and warm-season precipitation could enhance both drought and flood hazards in this economically vital region. Plain Language Summary The Mississippi River basin is an economically vital region for industry and agriculture, but it is prone to disruptive flooding and drought. Here we use organic molecules preserved in a lake sediment core to reconstruct the temperature and seasonal moisture patterns of the last 1,800 years. Our reconstruction shows that the Medieval period (ca. CE 1000-1,600) was relatively warm and that warmer temperatures in this region are associated with increased frequencies of severe droughts and floods. These findings generally support climate model projections of increasing drought and flood hazard in the Mississippi River basin as a result of anthropogenic climate change.

Severe Little Ice Age drought in the midcontinental United States during the Mississippian abandonment of Cahokia

“…However, estimated temperature changes of + / − 2 °C (see Fig. 2 D, E) and historical drought temperatures 31 are not large enough to explain the full 6.5‰ range in δ 18 O variability observed in the Horseshoe Lake sediment record. Taking into account the effects of temperature on δ 18 O includes a + 0.59‰ °C −1 influence on precipitation δ 18 O 32 and − 0.22‰ °C −1 for calcite forming from lake water (i.e., δ 18 O HSL ) 33 .…”

Reduced Lower Mississippi River Discharge During the Medieval Era

“…Gridded reconstructions of soil moisture (PDSI) based on tree-rings (E. R. Cook et al, 2010) show that aridity of the Medieval era was most pronounced across the central and western portions of the Mississippi River basin (Figure 3b), and that Medieval era droughts were more extreme than those of the period 1600-1800 CE (Figure 3c; E. R. Cook et al, 2004). Our paleohydrologic reconstruction shows that Q 2 increased after the Medieval era as midcontinental climate became cooler and wetter (Bradley & Jones, 1993;Mann et al, 2009;Muñoz et al, 2020), implying that regional changes in climate mediate discharge of the lower Mississippi River. More specifically, our findings point to an inverse relationship between median peak annual flows (Q 2 ) of the lower Mississippi River and mean annual temperatures, such that warmer temperatures result in decreased discharge over centennial time-scales.…”

“… Reconstructed climate on the Mississippi River basin: (a) Temperature (brGDGT) record from Horseshoe Lake, IL (Muñoz et al., 2020), normalized probability density functions of midcontinental (32°N‐46°N, 105°W‐90°W) megadrought events (≥10 consecutive years −0.5 PDSI (E. R. Cook et al., 2010) and Great Plains dune activation (Forman et al., 2008; Hanson et al., 2009, 2010), estimates of relative Q 2 discharge from meander wavelength. (b) Difference in reconstructed Mississippi River basin PDSI between the periods 1000–1200 CE and 1600–1800 CE (E. R. Cook et al., 2010).…”

“…10.1029/2020GL091182 5 of 10 and drier Medieval era (Muñoz et al, 2020) with severe and frequent midcontinental droughts (E. R. Cook et al, 2010) and increased aridity (Forman et al, 2008;Hanson et al, 2009Hanson et al, , 2010) (Figure 3a). Gridded reconstructions of soil moisture (PDSI) based on tree-rings (E. R. Cook et al, 2010) show that aridity of the Medieval era was most pronounced across the central and western portions of the Mississippi River basin (Figure 3b), and that Medieval era droughts were more extreme than those of the period 1600-1800 CE (Figure 3c; E. R. Cook et al, 2004).…”

“…Air temperature is a key atmospheric variable that regulates evapotranspiration and atmospheric moisture, and thus represents an important constraint on past and future changes in discharge. Here, we use a new paleohydrologic reconstruction together with model simulations that encompass the Medieval era (c. 1000–1200 CE), a period characterized by drought, aridity, and elevated temperatures across midcontinental North America (E. R. Cook et al., 2004; Herweijer et al., 2006; Muñoz et al., 2020), to evaluate the response of the lower Mississippi River to these regional changes in climate.…”