Global climate forcing on late Miocene establishment of the Pampean aeolian system in South America

Abstract

Wind-blown dust from southern South America links the terrestrial, marine, atmospheric, and biological components of Earth's climate system. The Pampas of central Argentina (~33°-39° S) contain a Miocene to Holocene aeolian record that spans an important interval of global cooling. Upper Miocene sediment provenance based on n = 3299 detrital-zircon U-Pb ages is consistent with the provenance of Pleistocene-Holocene deposits, indicating the Pampas are the site of a long-lived fluvial-aeolian system that has been operating since the late Miocene. Here, we show the establishment of aeolian sedimentation in the Pampas coincided with late Miocene cooling. These findings, combined with those from the Chinese Loess Plateau (~33°-39° N) underscore: (1) the role of fluvial transport in the development and maintenance of temporally persistent mid-latitude loess provinces; and (2) a global-climate forcing mechanism behind the establishment of large mid-latitude loess provinces during the late Miocene.

Fig. 1. Simplified geologic map of the Pampas and surrounding areas in South America.

The map includes Quaternary loess and sand deposits, the distribution of the upper Miocene Cerro Azul Formation (stippled region), and sample locations. Circles depict specific locations of loess and paleosol samples from the Cerro Azul Formation. Major regional rivers, including the Negro, Colorado, Desaguadero, and Paraná rivers are shown. Abbreviations are as follows, Chadileuvú block (CB), Pampa Central block (PCB), and San Rafael block (SRB). Quaternary sediment was transported from the Cordillera as well as uplifted foreland blocks through the Colorado, Negro, and Desaguadero rivers and entrained by westerly and southwesterly winds. Paleoclimate data referenced in the text were collected from the Sierras Pampeanas.

Fig. 2. Kernel density estimates (KDE) and histograms of U-Pb detrital zircon ages from the Cerro Azul Formation and Pleistocene–Holocene deposits.

A Composite KDE of the Cerro Azul Formation (this study) and Pleistocene–Holocene loess and loessic paleosol deposits in the Pampas. The Cerro Azul Formation and the Pleistocene–Holocene deposits contain the same detrital zircon age modes, with several of the largest age modes having similar proportions in both datasets. KDE are area-normalized and constructed with an Epanechnikov kernel using a bandwidth of 15 Myr. The number of grains within each 25 Myr histogram bin is shown on the y-axis. Fifty grains have ages older than 2 Ga and do not appear on the plot. B Composite KDE of the Cerro Azul Formation and the Pleistocene–Holocene deposits, excluding ages <5 Ma, which post-date deposition of the Cerro Azul Formation. KDEs are area-normalized and constructed with an Epanechnikov kernel using a bandwidth of 5 Myr. The base image is from Google Earth Pro using Landsat: Copernicus.

Fig. 3. Three-dimensional multidimensional scaling (MDS) plots of detrital zircon data.

A Samples reported in this study were plotted with Pleistocene–Holocene loess and loessic paleosols, river samples, Holocene dunes, Miocene sandstones, and potential sediment source regions. Solid lines connect samples to their closest neighbor and dashed gray lines represent the second closest neighbor. The overlap between upper Miocene and Pleistocene–Holocene samples indicates statistical similarity and similar sediment provenance. Data sources: 1, 11, 12 from ref. ; A-F from ref. ; G-K from ref. ; Dunes from ref. ; Sandstone from ref. ; Sierras Pampeanas from refs. ^,–; Chaco from refs. ^,; North and South Patagonia from refs. ^,; Puna from refs. ^–. B Geographic location (from Google Earth; earth.google.com/web) of samples used in (A).

Fig. 4. Summary of uplift, C₄ ecosystems, benthic foraminifera δ¹⁸O, and sea surface temperature estimates.

A A synthesis of elevation changes across the Tibetan Plateau where the present-day elevation is 100%. B C isotope-based construction of C₄ ecosystems. C Benthic foraminifera δ¹⁸O (‰ Pee Dee Belemnite) composite record from ref. showing the mid-Piacenzian Warm Period (mPWP) and mid Miocene Climate Optimum (mMCO). D Stacked $U_{37}^{K^{\prime }}$ sea surface temperature estimates from ref. . These include the Tropics, mid latitude Northern Hemisphere (Mid Lat NH; 30°–50°N), and mid latitude Southern Hemisphere (Mid Lat SH; 30°–50°N). Slope (m) calculated from linear regression through 5.50–8.75 Ma (i.e., the interval overlapping deposition of the Cerro Azul Formation) and 9.00–12.25 Ma. E–G) Elevation changes across the South American Cordillera including the Central Cordillera (~14°–27°S), South-Central Cordillera (~27°–34°S), and Northern Patagonia (~34°–48°S). Sources in Table S3.

Fig. 5. Simplified model of Hadley circulation in the Northern and Southern Hemisphere during the middle Miocene (ca. 15 Ma) and late Miocene (ca. 7–8 Ma).

Schematic distribution of lakes (light blue polygons), aeolian systems (light yellow polygons); and loessoid deposits (brown polygons). A Weak Hadley circulation extending into the East Asian continental interior; diffuse extended penetration of East Asian Summer Monsoon (EASM) precipitation^,. B Weak Hadley circulation extends south past the Pampas region. C Intensification and contraction of Hadley circulation, enhanced drying of the Asian continental interior, incursion of the East Asia Winter Monsoon EAWM;, and intensification and diminished penetration of the East Asian Summer Monsoon. D Intensification and contraction of Hadley circulation and establishment of the Pampas aeolian system. SALLJ, South American Low-Level Jet. The base images are from Google Earth Pro using Landsat: Copernicus.