A 10,000-year lake-sediment based reconstruction of precipitation isotope values in the Canadian Rocky Mountains
Bibliographic record
Abstract
We present a 10,000-year carbonate oxygen isotope (δ 18 O) record from Shark Lake, Alberta, (50.8411°N, 115.3983°W; 1,857 m above sea level) that provides insight on past changes in precipitation seasonality, regional hydroclimate dynamics and atmospheric circulation. Shark Lake has a hydrologically open configuration, with water isotope values that mirror annual mean precipitation, and contains a continuous sediment sequence consisting largely of authigenic carbonate minerals precipitated from the water column. The sediment δ 18 O record exhibits a shift from lower to higher-than-average δ 18 O values from the middle to late Holocene circa 4,100 calendar years before present (yr BP). This transition generally aligns with hypothesized transformations in atmospheric circulation that ostensibly led to reduced winter precipitation in northwestern North America and heightened aridity in the southwest during the late Holocene. Monte Carlo cross-correlation analysis demonstrates that other precipitation δ 18 O records from the Pacific Northwest have a significant positive correlation with the Shark Lake record, in contrast with records from the southern Rocky Mountains and central/eastern North America, which have a negative correlation with the Shark Lake record and others from the Northwest. We apply the previously established strong relationship between atmospheric circulation associated with the Pacific North American Pattern (PNA) and precipitation isotope values across North America to explain the marked coherency in precipitation δ 18 O reconstructions. Collectively, the paleo-records suggest a middle to late Holocene transition in the mean state of atmospheric circulation over North America from one similar to the negative phase of the PNA to one comparable to the positive PNA phase, in response to non-linear changes in ocean-atmosphere circulation driven by ice sheet decay and insolation forcing. This shift had substantial impacts on the source and isotopic composition of air masses traversing the continent and their interplay with circulation from the Gulf of Mexico as well as the seasonal amount and distributions of precipitation. Our results underscore the need for additional isotope-enabled climate model simulations and additional δ 18 O records, especially from understudied regions across North America, for a comprehensive view of seasonal-scale hydroclimate variations and synoptic-scale atmospheric circulation pattern changes over the Holocene. • 10,000-year δ 18 O record from Shark Lake reveals past hydroclimate changes. • δ 18 O records from across North America exhibit strong, significant correlations. • Northwestern δ 18 O records are anticorrelated with those from central/eastern regions. • A shift from negative to positive PNA-like circulation occurred in the mid-Holocene. • Air mass source and the amount and seasonal distribution of precipitation were impacted.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.004 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.001 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".