Fecal stanols show simultaneous flooding and seasonal precipitation change correlate with Cahokia’s population decline

Environmental Degradation and Disturbance.

Lopinot and Woods (16) and Woods (8) proposed that environmental degradation played a major role in Cahokia’s population decline, through deforestation and subsequent increased watershed erosion. However, palynological data indicate that much of the native forest was cleared at ∼450 CE (43), well before Cahokia’s population maximum in the 11th century. Although increased soil erosion is not the only cause of increased detrital mineral flux, it is anticipated that mineral content and mass accumulation rates would be higher during periods of maximum catchment disturbance related to mound building and agriculture. Mass accumulation rates of detrital minerals in HORM12 (43) are highest, predating the population maximum as defined by the fecal stanols, and drop abruptly in the 10th century. Mass accumulation rates are actually lowest during the population maximum and increase slowly as population declined (Fig. 2). Thus, increased soil erosion seems to occur as the region is depopulated, rather than immediately before. Although not definitive, this relationship suggests that if a link exists between anthropogenic disturbance and population dynamics, it is complex.

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Fig. 2.

HORM12 fecal stanol data (A) in relation to HORM12 flood events (6) (B), HORM12 loss-on-ignition-derived mineral content (43) (C), and HORM12 mass accumulation rates (56) (D).

Natural disturbance caused by Mississippi River floods may have contributed to Cahokia’s social reorganization and depopulation. HORM12 fecal stanol values indicate a population maximum before flood event V of Munoz et al. (6), whereas after the flood, the fecal stanol ratio is lower than at any preceding time in the record (Fig. 2). The stratigraphic position of the decline in fecal stanols relative to the flood supports Munoz et al.’s (6) association between the timing of massive flooding of the Mississippi River and late 12th century sociopolitical changes in the Cahokia area that are indicated by evidence for regional population decrease, agricultural contraction, palisade construction, a significant reduction in the creation of new mounds, final capping of some mounds, catastrophic abandonment of some sites, and the reorganization of residential space at Cahokia (2, 11, 13, 29, 43). The majority (∼70%) of historical flood crests on the Mississippi River at Saint Louis (adjacent to Cahokia) occur in April–June, when crops are particularly sensitive to moisture and temperature (44, 45). Floodwaters on low-gradient rivers such as the Mississippi recede gradually over days or weeks, amplifying the effect of inundation on croplands. Although an increase in the fecal stanol ratio from 100 to 50 cm indicates the population may have increased in the mid-16th to mid-18th centuries, it is unlikely that floods had a significant effect on these later populations because protohistoric and early historic groups in the region, such as the Tamaroas, were semisedentary; maize agriculture was less central to their subsistence than Mississippian groups; and they used the landscape in a different way than Mississippians (46, 47).

Hydroclimate Variation.

Despite the detailed information provided by both tree ring reconstructed PDSI measurements (4, 17) and δ¹⁸O values of endogenic carbonate from Martin Lake in Indiana (5), the direct relationship between hydroclimate and Cahokia’s regional population change has been obscured by chronological uncertainties of the associated age models and, in the case of Lake Martin, by distance to Cahokia. We present δ¹⁸O_c data from 15HSL as a proxy of hydroclimate to evaluate Benson et al.’s (4, 17) and Bird et al.’s (5) climate change hypotheses in relation to changes in Cahokia region population, as interpreted from fecal stanol data. We measured δ¹⁸O_c values on 44 15HSL sediment samples with multiple replicates. δ¹⁸O values ranged from −9.3‰ to −6.05‰ (SI Appendix, Table S1). Carbonates are not present in sufficient concentration for isotope analysis between 12 and 82 cm in 15HSL; thus, δ¹⁸O values provide only a partial picture of regional hydroclimatic change. δ¹⁸O values are moderately high during Cahokia’s population maximum and the initial drop from its peak (230–240 cm), alternating between approximately −6.5‰ and −7.5‰ (Fig. 3). δ¹⁸O values then rapidly decline to approximately −8.0‰ to −9.0‰ during Cahokia’s depopulation. The low δ¹⁸O values are interrupted by a positive excursion between 217 and 220 cm in core depth at the base of flood event V, and may signal the influx of river water with a different isotopic composition. Values remain low (average, approximately −8.5‰) from 217 to 187 cm (ca. 1150–1250 CE) before increasing steadily to a maximum value of −6.05‰ during Cahokia’s regional population low.

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Fig. 3.

15HSL fecal stanol data (blue) plotted against 15HSL δ¹⁸O data (orange). The horizontal lines mark a period of limited warm season precipitation, as interpreted from negative δ¹⁸O values.

The low values of δ¹⁸O_c during the population decline at Cahokia can be interpreted in multiple ways: as an increase in effective moisture; reduction in hydrologic residence time of the lake water, and hence a decrease in the evaporative modification of the lake water; or a shift in the mean weighted isotopic value of the lake water, resulting from a shift to more negative winter precipitation values. All these can work independently or in combination; however, any viable interpretation of the HSL δ¹⁸O_c record must be consistent with other proxy data. Tree ring reconstructions of PDSI from north and south of Cahokia show three multidecadal droughts from ∼1125 to 1250 CE (4). Modern tree growth in southern Illinois occurs primarily in the late spring and summer (48). The tree ring records that form the basis of PDSI reconstructions also reflect late spring and summer seasonality (4). We expect droughts of the magnitude described by Benson et al. (4) to be recorded as more positive δ¹⁸O values in the carbonates of Horseshoe Lake during Cahokia’s population decline, as evaporative modification of the lake water preferentially removes ¹⁶O.

However, the droughts are coincident with a marked decrease and sustained low δ¹⁸O_c values. To sustain low δ¹⁸O values during droughts requires either that the lake became hydrologically open to the Mississippi River or that the weighted δ¹⁸O value of the lake water decreased substantially. Maps reconstructed by Milner (2) indicate that in the early 19th century, Horseshoe Lake was connected by a tie channel to the Mississippi River and to other lakes in the floodplain. An isotopic measurement from the 19th century (SI Appendix, Table S1), when the lake was open to the Mississippi River, is similar to isotopic values from the peak population in the 11th century and the population minimum in the 15th century, suggesting that if a tie channel was present through the 19th century, it did not reduce the residence time of the lake water enough to decrease the δ¹⁸O value to that seen in the period of the population decline.

Alternatively, a shift in the relative contribution of winter precipitation is compatible with the model proposed by Bird et al. (5). The low δ¹⁸O_c values during population decline are similar in timing and duration to an initial decrease in δ¹⁸O_c at Martin Lake (Fig. 4). While acknowledging uncertainties from different chronologies, δ¹⁸O values from both sites are relatively high in the 11th and early 12th centuries CE, but reach a minimum in the early 13th century. δ¹⁸O values increase out of the 13th century and reach higher values in the 14th century, after which the two records decouple.

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Fig. 4.

Fecal stanol data (A and B) plotted against North American temperature reconstruction (C) (53), HORM12 flooding events (D) (6), regional PDSI reconstruction (E) (4), δ¹⁸O data from 15HSL (F) and Martin Lake (G) (5), and HORM12 mineral content (H) (43). The chronology was established by Munoz et al. (43); time in calendar years (CE) is plotted from older (Left) to younger (Right). The erections of palisades are marked by dashed vertical lines (4, 13).

The coincidence of decreasing stanol values (i.e., diminishing population size) with lower 15HSL δ¹⁸O_c values is consistent with Bird et al.’s (5) and Benson et al.’s (4) hypotheses that growing season drought was detrimental to maize agriculture, and thus the Cahokia population (Fig. 4). Seasonal drought documented in the PDSI reconstruction is consistent with changes to the seasonality of precipitation outlined by Bird et al. (5); drought in reconstructed PDSI reconstructed from tree rings coincides with a dominance of winter precipitation and loss of spring/summer rains inferred from the isotope data. The correlation of high fecal stanol and δ¹⁸O values at Horseshoe Lake supports Bird et al.’s (5) hypothesis that more positive δ¹⁸O values during the MCA represent climatic conditions conducive to supporting large Mississippian populations in the midcontinent.

The transition of 15HSL δ¹⁸O to more negative values occurred at ∼1150 CE (Fig. 4). The timing of this shift is significant because Benson et al.’s negative PDSI excursion and Munoz et al.’s (6) flood event V also took place at that time. After lows in the 13th century, δ¹⁸O values increase to a maximum at ∼1400 CE, despite a continued decline of fecal stanols. The increase in δ¹⁸O values may indicate a return to enhanced summer precipitation. Although this initially corresponds to a slight increase in δ¹⁸O values at Martin Lake, the lake shows a second century-long decrease (∼1375–1425 CE) that is delayed by several decades in the 15HSL record. The decoupling of the records can be explained by regional climatic variation, uncertainties in chronologies, or evaporative enrichment of Horseshoe Lake carbonates from a potential closure of the basin overpowering the effect of precipitation source.

MCA to LIA Transition Hypothesis.

The effect of climate change on human populations during the MCA and LIA is well documented worldwide (49⇓⇓–52). Previous studies (5, 6) noted that Cahokia developed during the warm MCA, but did not compare temperature reconstructions with a robust population proxy and other paleoenvironmental indicators. Cahokia’s population maximum coincided with warm reconstructed North American mean temperatures (53) during the 11th century. Temperature and fecal stanol values then decrease in unison until the 13th century (Fig. 4). The correlation of reconstructed North American mean temperatures and Horseshoe Lake fecal stanol trends implies that large-scale climate variability is an important contributor to population change at Cahokia.

Notably, the transition appears to have occurred rapidly and uniformly, as δ¹⁸O values, grain size, and tree-ring data all underwent major changes at ∼1150 CE. Thus, a major flood and reduced warm season precipitation occurred at the same time. Perhaps one environmental change would not have been consequential, but a combination of two or more changes would have presented significant challenges for a centralized, agrarian social system. Around this time, the inhabitants of Cahokia constructed a series of palisades that have been interpreted as indicators of societal stress (13), and several other lines of evidence indicate that there was a reorganization of Cahokia’s sociopolitical structure, including the destruction of outlying population centers, decline in construction of earthen monuments at Cahokia, shifts in the prestige goods economy, and a contraction of agriculture (2, 10, 13, 29, 43, 54). The construction of the first palisade at the time of flood event V, followed by repeated rebuilding during the period of decreased summer season precipitation, along with evidence for considerable change in other aspects of Cahokian society, implies that environmental events were significant factors synergistically associated with Cahokia’s population decline and reorganization.