Late Pleistocene Climate and Dust Source From the Mobarakabad Loess–Paleosol Sequence, Northern Foothills of the Alborz Mountains, Northern Iran

Abstract

Paleoclimatic investigation of loess-paleosol sequences from northern Iran is important for understanding past changes in a region highly sensitive to shifts in precipitation, and along potential routes of past human migration. Here, we present carbon and oxygen isotopic compositions of bulk carbonate (δ13Cbc and δ18Obc, respectively) coupled with particle size distributions of samples from the Mobarakabad section, northern Iran, to study past wind dynamics and hydroclimate. We also present new initial clay-sized Hf-Nd isotope results from key horizons in order to assess general dust sources. Variations of δ13Cbc and δ18Obc values of modern soils compared to paleosols allow reconstruction of late Pleistocene–Holocene climate change in the area. Our results show severe drought during a major eolian deposition phase (EDP) after 34 ka. The thickness and PSD of the C horizon of unit 5 suggest significant shifts in loess sources and depositional environments during this EDP after 34 ka. Indeed, based on our new clay-sized Hf-Nd data, we hypothesize that the loess unit 5 might originate from the young crustal source of the Alborz and Kopet Dagh mountains. In general, the PSD of C horizons in the section is bimodal in the silt fraction and the very small, very fine clay fraction, with a mode at c. 1 μm in the modern soil and paleosols possibly produced by weathering and pedogenic processes. There also appears to be a good correlation between δ13Cbc and δ18Obc values, differentiating phases of loess accumulation and paleosol formation and hence providing quantitative data for reconstructing paleoclimatic conditions in the study area.

Defining paleoclimatic routes and opportunities for hominin dispersals across Iran

“…Relatively well-developed paleosols formed in MIS 5a, 5c and especially 5e, with forest or steppe conditions and warmer, wetter climates, potentially driven by precessional insolation peaks, and MIS 5e likely warmer and wetter compared to today. Soil carbonate stable isotope and mineral magnetic analyses of the MIS 5e soil suggest particularly wet winters and hot dry summers [ 77 ], perhaps evidence of enhanced winter moisture input via the Westerlies. MIS 5c and 5a also show evidence for strong soil formation, indicating enhanced westerly winter moisture, with comparisons to Holocene soils suggesting similar conditions [ 72 ].…”

“…These climatic trends correspond with those observed in Lake Urmia [ 73 ], suggesting relatively coherent climatic dynamics across northern Iran during the Late Pleistocene. Perhaps the most striking feature in these loess records is the numerous phases of soil development occurring at multiple sites during MIS 3 and partly MIS 2, at the same time as relatively high loess accumulation rates [ 77 ]. The Neka-Abelou sequence shows these multiple soil formation episodes during MIS 3 ( Fig 4a ), though soil development is weaker than during MIS 5 and 1, the latter of which overprints MIS 2 loess.…”

“…Chronological uncertainties and differential preservation of soils at different loess profiles prevents specific wider correlations for these soils, but the fluctuations may be linked to millennial-scale hemispheric Dansgaard-Oeschger climate fluctuations, seen particularly in the Greenland ice cores, although they may also be influenced by changing water levels in the nearby Caspian Sea [ 74 , 75 ]. Overall though, despite these pluvial episodes during MIS 3, the loess records show a tendency to extreme drought during the last glacial period, particularly during MIS 4 and 2 [ 77 ]. Further east, close to the border with Turkmenistan and Afghanistan, loess-like deposits are also found covering a similar time interval and intercalated with fluvial sediments [ 71 ].…”

“…However, in light of comparable evidence from neighboring regions we propose that water availability, raw material resources, and favorable climatic conditions would have been primary drivers of these populations. The lack of temporally well-constrained environmental records and of fossil remains inhibits conclusions, however, tentative proxy and faunal paleontological evidence [ 61 , 77 , 81 , 82 ] suggest some humidity prior to MIS 5. It is thus possible that our proposed dispersal routes could also apply to hominin migrations during humid periods prior to MIS 5, however at present this remains unconfirmed.…”

Geophysical sediment properties of a late Pleistocene loess–paleosol sequence, Chenarli, northeastern Iran

“…Little information is available on the origin of loess in the area. Ghafarpour et al (2021b) pointed out that loess in the Mobarakabad section in the NFAM may have originated from actively eroding crustal sources from relatively young mountain belts (Alborz and Kopet Dagh) around the loess accumulation area. Furthermore, Költringer et al (2022) proposed a complex system for the primary sources and transportation pathways of loess at Aghband in the NILP, including sediments of the South Caspian Basin, the Karakum Desert, and nearby and distant mountain ranges.…”