Study Site

Martin Lake is a 17 meter-deep monomictic head-water kettle lake in northeastern Indiana (Figs 1 and S1). The lake is fed by ephemeral streams that merge into a single inflow on Martin Lake’s east side with additional hydrologic contributions from near surface groundwater (i.e., base flow)24. Year-round outflow drains to Olin Lake through a modified natural outlet on the lake’s western shore. Regional precipitation (910 mm yr−1) results in approximately 1.18 × 107 m3 of precipitation over the lake’s 12.86-km2 catchment, which is more than ten times Martin Lake’s volume (1.11 × 106 m3). This produces hydrologically open conditions and a short mean residence time of 103 days24. Despite its open hydrology, Martin Lake maintains persistent thermal stratification during the warm-season from April through November, which results in bottom water anoxia from March through December (Fig. S1)25. Also during the warm-season, biogenically mediated calcium carbonate is precipitated in the lake’s upper water column, which under anoxic conditions results in the accumulation of rhythmically repeating mm-scale couplets comprised of discrete detrital/organic matter and authigenic calcite laminae (Fig. S2).

Precipitation, Martin Lake Water and Authigenic Calcite δ18O

Event-based precipitation δ18O measurements from Indianapolis, IN (n = 98) collected between December 2014 and November 2015 define a regional meteoric water line (RMWL) with a slope of 7.9 and y-intercept of 9.4, consistent with the global meteoric water line (GMWL; Fig. 2) 26. The unweighted annual δ18O precip value of −8.3‰ is similar to modeled annual average δ18O precip for the region (−8.1‰)27 and to the seasonally weighted annual δ18O precip value (−8.0‰) determined from back trajectory analysis of the event-based δ18O precip measurements (discussed below). Surface and water column samples from Martin Lake (n = 44; average δ18O lw = −7.6‰) plot along the RMWL and are therefore consistent with meteoric waters (Fig. 2 and Table S1). Were Martin Lake influenced by evaporation, its surface waters would plot along the regional evaporation line (REL), which is defined by 449 surface water samples collected from lakes across Indiana between 2010 and 2015. Average Martin Lake δ18O lw and δD lw are instead within 0.2‰ and 1.4‰ of their respective REL-RMWL intercept values (Table S1). The correspondence between Martin Lake surface water isotope values and the REL-RMWL intercept indicates that Martin Lake water reflects the regional average isotopic composition of precipitation for Indiana, which includes a large portion of the lower Ohio River valley.

Figure 2: Hydrogen and oxygen isotope results for Indianapolis, IN, precipitation events (gray circles), Martin Lake, IN (red circles), Indiana lakes (light blue circles), and the White River, IN (green circles). The white box indicates the intersection between the REL and RMWL that defines average annual δ18O precip for Indiana. Full size image

Isotopic measurements of modern calcite from littoral vegetation at Martin Lake (−8.4‰) are identical within error to predicted δ18O cal (−8.5‰), given an average warm-season near-surface temperature of 18 °C25 and average δ18O lw of −7.6‰. This indicates that authigenic calcite precipitates in equilibrium with Martin Lake surface water and thus that down-core measurements of δ18O cal can be used as a reliable proxy for past changes in regional δ18O precip .

Modern δ18O precip Controls

Back trajectory and cluster analysis of the event-based precipitation samples were conducted with the US National Oceanic and Atmospheric Administration (NOAA) Hybrid Single-Particle Lagrangian Integrated Trajectories (HYSPLIT) model using the NOAA Global Data Assimilation System data28. Consistent with modern midcontinental climatology29, the results show that warm-season (Apr-Nov) rainfall accounts for 74.5% of mean annual precipitation and occurs mainly as discrete rainstorms when southerly mean atmospheric flow advects high δ18O precip (−5.5‰) from the Gulf of Mexico into the midcontinent (Fig. 3 and Table S1). Cold-season (Dec-Mar) precipitation accounts for 25.5% of annual totals and is sourced from Pacific and Arctic regions with characteristically low δ18O precip (−16.4‰), resulting in a moisture source δ18O difference of 10.9‰. The average δ18O precip , weighted to reflect warm and cold-season contributions, equals −8.0‰. This is consistent with the REL-RMWL intercept, modeled δ18O precip and average Martin Lake δ18O lw , further supporting that Martin Lake δ18O lw reflects regional climate.

Figure 3 Composite mean maps of (A) cold-season (Dec-Mar) and (B) warm-season (Apr-Nov) 850 hPa vector winds between 1949–2015 from the NCEP-NCAR reanalysis database. Arrows are wind directions with colored shading indicating velocity (m/s). Also plotted are mean back trajectories for cluster 1 (thick white line in A) and cluster 2 (thick red line in B) of the event-based Indianapolis, IN, precipitation samples. White squares are spaced at 24 hr intervals. White circles indicate the starting point from which back trajectories were calculated. The mean back trajectories are in good agreement with their respective seasonal atmospheric circulation patterns since 1949, indicating the precipitation back trajectories are representative of the modern climatological mean. Images provided by the NOAA/ESRL Physical Sciences Division, Boulder Colorado (http://www.esrl.noaa.gov/psd/). Full size image

Changes in the proportion of northerly and southerly moisture sources strongly affect δ18O precip across the midcontinental US and are linked to the PNA30, the leading mode of continental US climate variability on seasonal and longer timescales31. During negative (−) PNA phases, the atmospheric circulation pattern increases midcontinental precipitation originating from 18O-enriched southerly moisture sources30,31,32, and reduces the amount of precipitation in the southwestern US. Conversely, during positive (+) PNA phases, the frequency of 18O-depleted northwesterly air mass incursions into the midcontinental US increases. As a result, the precipitation amount is reduced and δ18O precip values are lower in the Midwest during +PNA phases, while portions of the western US become wetter. On decadal and longer timescales, the PNA is modulated by the Pacific Decadal Oscillation (PDO)30,33, which is the multi-decadal, North Pacific expression of the El Niño-Southern Oscillation34, such that +PDO events are typically concurrent with a +PNA phase, and vice versa.

Relevant to this investigation, the strongest PNA-precipitation correlations in North America are located over Indiana and the central Mississippi and lower Ohio River valleys, with correlations (r) as high as −0.71 (Fig. S4)32. Similar precipitation-PNA correlations (based on data from 1949 and 2015 CE) exist on seasonal (April-August) and annual timescales, demonstrating a consistent long-term PNA-hydroclimate relationship in the midcontinental US (Figs 1 and S4). Paleoclimate records from the midcontinental US that can be linked to PNA variability are therefore useful for investigating regional hydroclimate patterns associated with this teleconnection and their relationship with Pre-Columbian Native American cultural changes.

Late Holocene Midcontinental Hydroclimate Variability

Continuous down-core δ18O cal measurements with a 5-year resolution from Martin Lake show considerable variability and a 12.5‰ range (Fig. 4). Temperature is unlikely to have contributed substantially to δ18O cal variability because the combined influence of temperature-controlled isotopic fractionation of liquid water in the air mass (+0.59‰ °C−1)26 and calcite from lake water (−0.22‰ °C−1)35 would require an unrealistically large change in temperatures (e.g., a change of approximately 34 °C is needed to explain the full range). Instead, we contend that the 12.5‰ range in Martin Lake δ18O cal reflects mean state changes in the predominance of northerly and southerly moisture sources, which have an isotopic difference of 10.9‰ (Fig. 1). Because the PNA exerts strong control on regional midcontinental δ18O precip 30, we interpret periods with high δ18O cal to reflect sustained −PNA-like phases at Martin Lake, as well as over the central Mississippi and lower Ohio River valleys, leading to high δ18O precip and δ18O cal values. Conversely, +PNA-like phases promoted cold-season-like northerly atmospheric flow over the midcontinental US, resulting in low δ18O precip and δ18O cal values. This interpretation is supported by the strong correlation between Martin Lake δ18O cal and a reconstruction of the winter PNA index (wPNA) from 1725 to 1840 CE based on 12 tree-ring records from the continental US and Alaska36. The Martin Lake-wPNA δ18O correlation (r = −0.70, p < 0.001; Fig. 4) is similar to those of the modern PNA-precipitation relationship32, thus further supporting the use of down-core Martin Lake δ18O cal measurements as an indicator of synoptic-scale PNA-like variability. The Martin Lake record is strongly impacted by regional land clearance after the 1840 s37, with increased deposition from watershed erosion in response to deforestation and greater catchment discharge38. Therefore, the weakened post-1840 CE wPNA-Martin Lake δ18O cal correlation is interpreted to reflect a change in watershed hydrology as a result of large-scale land clearance, rather than a disconnect between δ18O precip and the PNA. Nonetheless, high average δ18O cal at Martin after 1840 CE is in line with a generally negative PNA mean-state during the last two centuries36, and together with the strong pre-1840 correlation, supports interpreting down-core δ18O cal in terms of mean state changes in δ18O precip related to PNA-like variability.

Figure 4 (A) Comparison of Martin Lake δ18O cal with reconstructed winter PNA variability since 1725 CE. Dark gray shading indicates undisturbed laminated sediments. Light gray shading designates mottled sediments reflecting watershed land clearance. (B) Simplified Martin Lake stratigraphic profile. Lined sections represent laminated sediments, gray shading indicates mottled sediments. Martin Lake results for (C) δ13C cal , (D) δ18O cal, and (E) % lithics (y-axis truncated at 60%) plotted against (F) water level reconstructions at Hole Bog, MN, (purple) & Minden Bog, MI, (orange), (G) tree-ring based western US drought percentage (30-year low-pass filtered) and (H) the Moon Lake diatom-inferred salinity index. Shaded vertical boxes define midcontinental pluvials (blue), droughts (red) and the midcontinental (MC) abandonment between 1350–1450 CE (gray). Colored horizontal lines indicate time series averages. Black triangles designate 14C dates. Dashed purple line in F demarcates the Hole Bog LIA drought hiatus. Full size image

Changes in the relative contributions of moisture sources were estimated using a two-component δ18O precip mixing model with end-member δ18O precip values representing northwesterly (−16.4‰; +PNA) and southerly (−5.5‰; −PNA) moisture sources that were scaled to match past δ18O cal variability during specific time intervals (Tables S1 and S2). High δ18O cal between 950 and 1190 CE suggests that −PNA-like conditions prevailed at Martin Lake and over the central Mississippi and lower Ohio River valleys during the MCA, with southerly warm-season rainfall accounting for approximately 68% of annual precipitation during early population expansion among the Mississippian, Fort Ancient and Monongahela cultures (Fig. 4). Archaeological data show infrequent signs of warfare-related trauma and minimal construction of city center fortifications during this time, suggesting relatively low levels of conflict11,13. Decreasing δ18O cal between 1190–1250 CE marks a shift to +PNA-like conditions in the midcontinental US that persisted throughout the LIA. This time period was characterized by approximately equal distributions of warm and cold-season precipitation and coincides with a rapid rise in fortification construction around Mississippian cities. Subsequently, an especially pronounced δ18O cal minimum between 1400–1470 CE marks the strongest +PNA-like phase of the last 2100 years, contemporaneous with the final abandonment of midcontinental Mississippian cities in the central Mississippi and lower Ohio River valleys. During this timeframe, midcontinental precipitation was largely from northerly cold-season sources (79%) and summer precipitation was substantially reduced. By 1500 CE, these same and related Mississippian populations were concentrated in the lower Mississippi River valley and interior of the Southeast where they encountered the Spanish during the 16th century11,20. Modern seasonal precipitation distributions were established after 1830 CE, as climate transitioned toward the current warm period (CWP; 1900 CE to present).

The strong covariance between Martin Lake δ18O cal and δ13C cal data (r = 0.81; p < 0.001; Fig. 4) supports the inferences on Pre-Columbian precipitation seasonality derived from the δ18O cal record alone. Prolonged warm-season thermal-stratification in midcontinental lakes similar to Martin Lake increases the sequestration of 13C-depleted organic matter in anoxic bottom water, leaving surface water enriched in 13C and resulting in high δ13C cal 39. Because midcontinental warm-season precipitation is dominated by southerly moisture sources with high δ18O values, both δ13C cal and δ18O cal increase during extended warm-seasons39. The opposite occurs when climate is colder and moisture is predominantly sourced from northerly regions.

Prolonged stratification during the early expansion of late Pre-Columbian populations (950–1200 CE) indicates extended warmth that was associated with southerly sourced moisture. This is consistent with warmer Midwestern summers40 and above-average Northern Hemisphere temperatures41 during the MCA, suggesting the δ13C cal values reflect not only warmer temperatures at Martin Lake, but regionally across the midcontinental US (Fig. 5). In contrast, reduced stratification during the LIA when moisture was sourced from northerly regions indicates shorter warm-seasons, especially between 1400–1470 CE. These trends were associated with cooler Northern Hemisphere and Midwestern summer temperatures, supporting regionally cooler midcontinental temperatures during the LIA with peak cooling between 1400–1470 CE40 when the Vacant Quarter was established in the central Mississippi and lower Ohio River valleys (Fig. 5).

Figure 5 PNA variability inferred from Martin Lake (A) δ18O cal (black) and δ13C cal (red) compared with (B) PDO and (C) Northern Hemisphere temperatures during the last 1500 years. (D) Martin Lake δ13C cal is compared with a multi-site pollen-based Midwestern summer temperature reconstruction, showing that thermal stratification closely follows regional temperature variability. Shaded boxes, horizontal lines and black triangles as defined in Fig. 4. Full size image

The concentration of lithic material (the inorganic clastic sediment fraction) in Martin Lake sediments (Fig. 4) provides a record of terrestrial erosion resulting from warm-season precipitation events critical for the non-irrigated maize agriculture practiced by the Mississippians and related late Pre-Columbian populations. Sediment delivery to Martin Lake occurs predominantly during the warm-season when the lake is ice-free and abundant rainfall from southerly derived rainstorms mobilizes watershed sediment. Greater warm-season precipitation, i.e., more frequent rainstorm events, would therefore result in greater sediment delivery to Martin Lake; drier conditions would have the opposite effect. This interpretation is consistent with the co-occurrence of high lithic concentrations during the MCA when δ18O cal and δ13C cal values were also elevated, indicating enhanced warm-season rainfall and watershed erosion during warm, extended −PNA-like conditions. Conversely, low lithic abundances coincide with low δ18O cal and δ13C cal during the LIA, indicating reduced warm-season rainfall when cold-season conditions predominated under a more persistent +PNA-like mean state.

Although the lithics results are reflective of local precipitation over the Martin Lake watershed, they indicate changes in the frequency of southerly derived rainstorms, which follow trajectories that track over the central Mississippi and lower Ohio River valleys (Fig. 3). Changes in the frequency of these rainstorms therefore would impact areas along warm-season storm trajectories, including the central Mississippi and lower Ohio River valleys. This is reflected in the modern relationship between midcontinental rainfall and the phase of the PNA (i.e., reduced during +PNA phases and increased during −PNA phases; Fig. 1)32. The Martin Lake lithics results therefore suggest that regions which today experience increased warm-season precipitation during −PNA phases were wetter (i.e., more frequent southerly derived warm-season rainstorms) during the MCA, but drier during the LIA when +PNA-like conditions predominated. Minimum lithics concentrations between 1350–1450 CE during the strongest +PNA-like phase of the last 2100 years, indicate an especially pronounced PNA-related warm-season drought during the Mississippian abandonment of the midcontinent.

The occurrence of synchronous synoptic- and local-scale PNA-like hydroclimate variability in the midcontinental US suggested by the Martin Lake δ18O cal , δ13C cal and lithic data are supported by other regional paleoclimate records including amoeba testate water-table reconstructions from Minden (Michigan) and Hole Bog, MN (Mississippi headwaters; Fig. 4)42. These records, from locations that are today similarly affected by PNA variability, demonstrate generally elevated water tables during the MCA and CWP (−PNA). Conversely, water tables were lower at both sites during the LIA between 1250 and 1800 CE (+PNA) with a depositional hiatus at Hole Bog during this time indicating a dry climate in the upper Mississippi watershed. These results are in good agreement with the Martin Lake record and indicate that midcontinental sites separated by as much as ~1000 km were similarly and synchronously affected by PNA-like hydroclimate variability during the MCA, LIA and CWP. This supports the synoptic-scale nature of the hydroclimate variability captured by the Martin Lake record and its extrapolation to adjacent regions in the central Mississippi and lower Ohio River valleys (~500 km from Martin Lake), where midcontinental Mississippians and related Pre-Columbian cultures were centered.

The strong evidence for regional midcontinental drought after 1250 CE contradicts assertions of a wetter LIA, and more specifically that suspected increases in flooding at Cahokia after 1200 CE were indicative of larger-scale flooding that contributed to the broader Mississippian abandonment of the midcontinent23. Cahokia’s location below the confluence of the Missouri and Mississippi Rivers complicates interpretations of flood records from this location, in that attribution to a specific river drainage is not possible, thus preventing any firm conclusion regarding the nature of climate change and the putative flood packages recorded in the American Bottom23. We therefore suggest that if increased flooding did indeed occur at Cahokia after 1200 CE, it reflects increased discharge from the Missouri River, for which the majority of the drainage basin is located in the western US. This assertion is consistent with records that indicate greater warm-season precipitation across much of the western US during the LIA, including the Missouri River drainage5,43,44, while midcontinental conditions were drier.

The PNA-like mean state changes in midcontinental hydroclimate variability identified in the Martin Lake and regional paleoclimate records are supported on a continental-scale by tree-ring drought reconstructions5, wild-fire histories44, and lake salinity43 records from the western US (Fig. 4). Regionally coherent western warm-season drought and increased fire occurrence during the MCA, when midcontinental records indicate pluvial conditions, mirrors modern east-west antiphased PNA-hydroclimate correlations (Fig. 1). The opposite conditions occurred during the LIA, consistent with a reversal in the US hydroclimate gradient. These paleoclimate inferences support the idea of a persistent east-west dipole pattern in warm-season precipitation across the continental US that closely resembles modern warm-season PNA-precipitation relationships and, furthermore, that PNA-like hydroclimate variability has affected the distribution of pluvial and drought conditions on multi-decadal to centennial timescales during the last 2100 years.

Consistent with Pacific atmosphere-ocean modulation of the PNA and North American droughts, the PNA-like hydroclimate anomalies indicated by the Martin Lake and regional paleoclimate data largely track reconstructed PDO variability during the last 1500 years (r = 0.51, p < 0.01 for 15-yr moving averages of each time series; Fig. 5)4,33,41. Negative PNA-like mean states correspond to generally negative PDO-like conditions (the MCA) and vice versa (the LIA). The most prominent +PNA events recorded at Martin Lake (850–950, 1400–1470 and 1800–1820 CE) correspond to +PDO phases. Other δ18O cal minima (+PNA) also correspond with +PDO events, but with differences in the δ18O cal response magnitude (Fig. 5).

Hydroclimate and Midcontinental Native American Cultural Trends

Late Pre-Columbian population growth between 1000 and 1200 CE was associated with extended warm-season conditions during the MCA that were accompanied by above average precipitation derived from southerly moisture sources (Fig. 6). These climatic conditions would have been favorable for the adoption of intensive swidden agriculture, maize in particular, which enabled population growth and sustained emerging population centers. This scenario is consistent with modeling results21 that demonstrate that early agriculture would have been positively impacted by increased precipitation during late Pre-Columbian population expansion, when climatic conditions were favorable to growth and aggregation. An increased proportion of juveniles occurred in large skeletal assemblages from eastern North America dating to this timeframe45, suggesting elevated crude birth rates, consistent with more abundant and reliable food sources that positively impacted female reproductive ecology.

Figure 6: Martin Lake lithics as an indicator of warm-season rainfall (blue line) compared with human skeletal δ13C of midcontinental Pre-Columbian populations that reflects diet (red box and whisker plots), the percentage of fortified Mississippian sites as an indicator of conflict (black line and ovals) and notable events during the rise and fall of midcontinental human populations. Shaded boxes represent the adoption of maize agriculture (gray box with star) and the early (E; green), middle (M; orange) and late (L; red) Mississippian periods, and the post-historic period (PH; light gray). The dashed box marks the 1350–1450 CE midcontinental abandonment. Human skeletal δ13C values after 1450 CE are from sites located outside of the Vacant Quarter. Full size image

The widespread adoption of maize agriculture by midcontinental Native American populations during the early MCA is confirmed by a newly compiled regional database of human skeletal δ13C comprised of more than 1300 individuals from across the eastern US that reflects dietary changes during the last 2300 years (Figs 6 and S5 and Table S3). An abrupt increase in human skeletal δ13C from background values of −20.3‰ (350 BCE to 950 CE) to −15.9‰ between 950–1050 CE at sites across the Midwest and into Pennsylvania reflects a regional dietary change from one relying exclusively on C3 plants to one where 50% or more of protein was provided by C4 plants (i.e., maize)45. This supports the hypothesis that expanding populations, aggregation and social complexity were fueled by the rapid and widespread adoption of intensive maize agriculture between 950 and 1050 CE made possible by an opportune climate12,13.

The signatures of warfare and conflict, including skeletal trauma and active defensive features (i.e., palisades with bastions), increased markedly after 1150 CE across the midcontinent (Fig. 6)11. Consistent with the modeled climate-agriculture-population response12, these trends mirrored declining warm-season precipitation during the transition to the LIA, the very same period when maize consumption peaked among Native American populations (i.e., human skeletal δ13C values from −10.3 to −10.1‰ between 1150–1450 CE). As LIA aridity became the new mean climatic state, farmsteads and hamlets gave way to larger, aggregated villages in many major river valleys, suggesting that safety superseded other necessities and constrained resource catchment areas18. In the central Illinois River valley, for example, several fortified villages show evidence for full-scale conflagration between 1200 and 1350 CE, alluding to frequent raiding and competition for declining resources. Furthermore, paleodemographic analyses of skeletal samples from this region demonstrate an increasing age-specific risk of death that was particularly severe among reproductive-age Mississippian females, likely because pathogen loads and disease transmission were higher in “compact village life” conditions, which slowly eroded the reproductive ecology and sustainability of these villages46. The progressive collapse of Mississippian populations between 1350–1450 CE coincides with exceptionally severe warm-season drought and extended cold-seasons. Shorter and drier growing seasons during this time would have significantly reduced agricultural yields, thereby contributing to intensified resource-related conflict that ultimately undermined the socio-political fabric and population dynamics of midcontinental Mississippian societies10,11,12,13,22.

Native American populations in the upper Ohio River Valley associated with the Monongahela and Fort Ancient cultures, as well as Mississippian groups in the southeastern and eastern US, persisted after depopulation in the lower Ohio and central Mississippi River valleys. While it is not clear exactly how these populations in the upper Ohio River Valley persevered through the LIA-associated drought episodes, the spatial and temporal patterning of settlements indicate that they were less ubiquitous after 1500 CE, with significant buffer zones between communities18,20. There are also indications that Fort Ancient villages shifted from year-round to seasonal occupation during the LIA, with an increased emphasis on hunting, including bison, to potentially cope with food production shortfalls and reduced maize consumption17,47. These smaller, constrained populations may have persevered due to 1) local climatic conditions, 2) a reduced population size that could be supported by floodplain agriculture alone, or 3) local socio-political autonomy that was more resilient and responsive to drought-induced resource stress. In the southeastern and eastern US (e.g., New York, the Ontario Peninsula, and regions along the east coast and lower drainages of the Delaware and Susquehanna Rivers), the persistence and even thriving, of Native American populations during and following the midcontinental abandonment20 is consistent with a decreasing PNA influence on climate with increasing distance from the Ohio River Valley32, as illustrated by the attenuated variability in paleoclimate records spanning the last millennium as compared with the Martin Lake results48,49,50.