How the Mississippi River Drained During the Late Pleistocene

The ancestral Mississippi drainage archived in the late Wisconsin Mississippi deep-sea fan

Authors:

Fildani et al

Abstract:

The response of continental-scale drainage systems to short-term (i.e., millennial-scale) climate change is unknown but has wide implications for understanding climate feedbacks and terrestrial-marine fluxes. The late Wisconsin Mississippi River to deep-sea fan of North America was one of Earth's largest sediment-routing networks during the most recent glacio-eustatic cycle. To understand late Pleistocene sediment production and dispersal related to the partly glaciated, ancestral Mississippi system, we sampled late Wisconsin deep-sea fan channel-fill and lobe deposits for detrital zircon U-Pb and (U-Th)/He double-dating analyses, from Deep Sea Drilling Project (Leg 96) cores and U.S. Geological Survey piston cores. Our results suggest a late Pleistocene glacial Mississippi system that forced a larger transfer of sediment from Cordilleran magmatic provinces and the Canadian Shield when compared to the modern drainage. This indicates a potentially more expansive and/or erosive ancestral Mississippi catchment, and the efficient dispersal of terrigenous sediment, nutrients, and solutes into the deep-sea via high-discharge meltwater and glacial-lake outbursts during ice retreat.

Academic Bun FIGHT! Questioning Cahokia's Fall Through a Great Flood

Great Flood?  Not the Cause!

Correlation does not equal causation: Questioning the Great Cahokia Flood

Authors:

Baires et al

Abstract:

Munoz et al. (1) present an argument for Cahokia’s demise focusing on a massive flood event ca. A.D. 1200. Although we recognize that a flood event may have occurred, we do not agree with their hypothesis that this event (i) caused the collapse of the largest Pre-Columbian city north of Mexico and (ii) occurred ca. A.D. 1200. Cahokians did not vacate the floodplain until after the mid-14th century, preceded by new communities and social relationships during the 11th to 13th centuries. We suggest the authors reconsider their data in conjunction with the prolific evidence for community persistence and revitalization initiated during the mid-12th century.

Damnit!  Yes, it did!

Reply to Baires et al.: Shifts in Mississippi River flood regime remain a contributing factor to Cahokia’s emergence and decline

Authors:

Munoz et al

Abstract:

Baires et al. (1) critique two aspects of our study (2): (i) whether a large flood event “caused the collapse of the largest Pre-Columbian city north of Mexico,” and (ii) the timing of one flood event (Flood Event V), which we reported to have a 95% confidence interval of A.D. 1100–1260. Instead, Baires et al. suggest Event V occurred during “several wetter periods during the late 13th to 15th centuries.”

To the first point, our paper never attributes Cahokia’s “collapse” to a single flood event. Instead, we show a temporal correspondence among midcontinental aridity, reduced megaflood frequency, and Cahokia’s emergence. We discuss the potential effects of floods on prehistoric populations in the floodplain and conclude that extensive flooding could have temporarily or permanently transformed Cahokia’s sociopolitical system. We note several independent lines of evidence that point to population decreases at Cahokia and the reorganization of its sociopolitical structure around A.D. 1200. We do not argue that Cahokia collapsed at this time, nor do we argue against community persistence after A.D. 1200. Therefore this first critique seems to be based on an oversimplified characterization of the arguments presented in our paper.

Cahokia Rise and Fall Coincided With Flood Frequency Shifts on Mississippi River

Cahokia’s emergence and decline coincided with shifts of flood frequency on the Mississippi River

Authors:

Munoz et al

Abstract:

Here we establish the timing of major flood events of the central Mississippi River over the last 1,800 y, using floodwater sediments deposited in two floodplain lakes. Shifts in the frequency of high-magnitude floods are mediated by moisture availability over midcontinental North America and correspond to the emergence and decline of Cahokia—a major late prehistoric settlement in the Mississippi River floodplain. The absence of large floods from A.D. 600 to A.D. 1200 facilitated agricultural intensification, population growth, and settlement expansion across the floodplain that are associated with the emergence of Cahokia as a regional center around A.D. 1050. The return of large floods after A.D. 1200, driven by waning midcontinental aridity, marks the onset of sociopolitical reorganization and depopulation that culminate in the abandonment of Cahokia and the surrounding region by A.D. 1350. Shifts in the frequency and magnitude of flooding may be an underappreciated but critical factor in the formation and dissolution of social complexity in early agricultural societies.

Evidence From Gulf of Mexico Deep Core Suggests Monsoonal Cycle was Intensified During Holocene Thermal Maximum

PALEOENVIRONMENTAL AND PALEOCLIMATIC IMPLICATIONS OF ENHANCED HOLOCENE DISCHARGE FROM THE MISSISSIPPI RIVER BASED ON THE SEDIMENTOLOGY AND GEOCHEMISTRY OF A DEEP CORE (JPC-26) FROM THE GULF OF MEXICO

Authors:

TRIPSANAS et al

Abstract:

Several meltwater floods initiated by the Laurentide Ice Sheet (LIS) drained to the Gulf of Mexico (GOM) through the Mississippi River during the last deglaciation (10–21 cal ka [calibrated kiloannum]). Such floods have been efficiently captured in the geochemical record of the Jumbo Piston Core 26 (JPC-26) from the northwestern GOM as distinct peaks in the content distribution of Si, Al, and Fe due to their common occurrence throughout North America. On the other hand, peaks in the content distribution of Ti, K, Zr, and V adequately describe the sediment source of each flooding event due to their clustered areal distribution in North America. The presence of three distinct peaks in the distribution of Si, Al, and Fe at 8.5, 5.2–7.6, and 2.9–3.5 cal ka indicates that events of enhanced Mississippi River discharge (EMRD) occurred during the Holocene as well. The geochemical signature of these flooding events suggests that their origin and sediment source is variable, depending on the retreating state of the LIS and the prevailing precipitation patterns in North America during these time periods. A comparison of the 8.5 and 2.9–3.5 cal ka EMRD events with the oxygen isotope and gas (CH4) records of the Greenland Ice Sheet Project 2 (GISP-2) ice core denotes that these events might be linked to short global warming episodes associated with periods of increased solar irradiance. The 5.2–7.6 cal ka EMRD event is associated with the Holocene Thermal Maximum, which resulted in the intensification of the North American monsoonal circulation.