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Abstract Understanding the hydroclimate representations of precipitation δ 18 O (δ 18 O p ) in tropical South America (TSA) is crucial for climate reconstruction from available speleothem caves. Our preceding study (Part I) highlights a heterogeneous response in millennial hydroclimate over the TSA during the last deglaciation (20-11ka before the present), characterized by a northwest-southeast (NW-SE) dipole in both rainfall and δ 18 O p , with opposite signs between central-western Amazon and eastern Brazil. Mechanisms of such δ 18 O p dipole response are further investigated in this study with the aid of moisture tagging simulations. In response to increased meltwater discharge, the Intertropical Convergence Zone (ITCZ) migrates southward, causing moisture source location shift and depleting the isotopic value of the vapor transported into eastern Brazil, which almost entirely contributes to the δ 18 O p depletion in eastern Brazil (SE pole). In contrast, moisture source location change and local condensation change (due to the lowering convergence level and increased rain reevaporation in unsaturated sub-cloud layers) contribute nearly equally to the δ 18 O p enrichment in the central-western Amazon (NW pole). Therefore, although a clear inverse relationship between δ 18 O p and rainfall in both dipole regions seems to support the “amount effect”, we argue that the local rainfall amount only partially interprets the millennial δ 18 O p change in the central-western Amazon, while δ 18 O p does not document local rainfall change in eastern Brazil. Thus, the paleoclimate community should be cautious when using δ 18 O p as a proxy for past local precipitation in the TSA region. Finally, we discuss the discrepancy between the model and speleothem proxies on capturing the millennial δ 18 O p dipole response and pose a challenge in reconciling the discrepancy.
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This content will become publicly available on May 1, 2027
Triple Oxygen Isotope Record From Ferricrete Deposits Shows Holocene Moisture Source Shift in the Rocky Mountains
Abstract Holocene hydroclimate changes in the western US are poorly constrained, in part because the region's complex topography and seasonally distinct circulation patterns can generate heterogenous responses over small spatial scales. To better understand regional precipitation patterns over the Holocene, we measured the triple oxygen isotopes of goethite from high‐elevation ferricrete deposits across a latitudinal gradient in the Rocky Mountains. Ferricretes form in stream water that reflects the isotopic composition of local precipitation, providing a record of past hydroclimate changes. We also used a 1‐dimensional vapor transport model to simulate precipitation δ18O and Δ′17O along a hypothetical storm track, given varying moisture recycling amounts and evaporation versus transpiration contributions. We find that both northern (central Montana) and southern (southwest Colorado) sites exhibit similar increases in goethite δ18O over time, accompanied by declines in goethite Δ′17O. To interpret these trends, we considered three potential drivers: temperature, moisture recycling and evapotranspiration partitioning, and moisture source. Δ′17O is largely insensitive to temperature, and our model shows that changes in moisture recycling and evapotranspiration partitioning along the storm track cannot account for the observed Δ′17O decreases. Instead, we conclude that the isotopic trends primarily reflect an increased contribution of southerly‐sourced precipitation, since moisture from the Gulf of Mexico—the dominant source of interior western US summer precipitation—is characterized by high δ18O and low Δ′17O relative to Pacific moisture. More generally, this work demonstrates how triple oxygen isotopes can be used to disentangle changes in temperature, atmospheric moisture dynamics, and moisture source in semi‐arid to arid regions.
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- Award ID(s):
- 2303484
- PAR ID:
- 10712630
- Publisher / Repository:
- AGU/Wiley
- Date Published:
- Journal Name:
- Paleoceanography and Paleoclimatology
- Volume:
- 41
- Issue:
- 5
- ISSN:
- 2572-4517
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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