Journal: EarthArXiv

Abbreviation

Publisher

EarthArXiv

Journal Volumes

ISSN

Description

Filters

2020 - 20242
Reset filters

Search Results

Publications 1 - 2 of 2
  • Modest, not extreme, northern high latitude amplification over the Mid to Late Miocene shown by coccolith clumped isotopes
    Item type: Working Paper
    Mejia Ramirez, Luz Maria; Bernasconi, Stefano M.; Fernandez, Alvaro; et al. (2024)
    EarthArXiv
    The ongoing global warming is characterized by a high latitude amplification effect, with Northern Hemisphere air temperatures increasing significantly faster than the global average. Widely-used paleotemperature proxies suggest that during past warm climate states, there was extreme high-latitude and polar amplified warming, along with flat latitudinal sea surface temperature (SST) gradients. Because these features remain difficult to simulate in climate models for periods like the Miocene, not only model construction, but also absolute values of proxy temperature estimates should be continuously revised. Clumped isotope thermometry is a tool that has the potential to bypass some of the limitations of other proxies, such as reliance on assumptions of past seawater chemistry, and other unknown mechanisms influencing their response to temperature changes. Here we provide the first downcore reconstruction of calcification temperatures from coccolith clumped isotopes (∆47) at northern high latitudes. This record shares trends with alkenone SSTs from the same samples estimated via widely-used calibrations, but suggest an on average ~9 °C colder North Atlantic over the last 16 million years (My). Coccolith ∆47 calcification temperatures agree better than alkenone-derived records with model simulations for the Mid and Late Miocene. If confirmed by additional records, a modest, rather than an extreme northern high latitude warmth, would entail paradigm-changing implications in our understanding of high latitude thermal response to anthropogenic CO2, while implying a need for revision of the present interpretations of currently considered well-validated temperature proxies like alkenone unsaturation ratios.
  • Global models for short-term earthquake forecasting and predictive skill assessment
    Item type: Working Paper
    Nandan, Shyam; Kamer, Yavor; Ouillon, Guy; et al. (2020)
    EarthArXiv
    We present rigorous tests of global short-term earthquake forecasts using Epidemic Type Aftershock Sequence models with two different time kernels (one with exponentially tapered Omori kernel (ETOK) and another with linear magnitude dependent Omori kernel (MDOK)). The tests are conducted with three different magnitude cutoffs for the auxiliary catalog (M3, M4 or M5) and two different magnitude cutoffs for the primary catalog (M5 or M6), in 30 day long pseudo prospective experiments designed to forecast worldwide M ≥ 5 and M ≥ 6 earthquakes during the period from January 1981 to October 2019. MDOK ETAS models perform significantly better relative to ETOK ETAS models. The superiority of MDOK ETAS models adds further support to the multifractal stress activation model proposed by Ouillon and Sornette (2005). We find a significant improvement of forecasting skills by lowering the auxiliary catalog magnitude cutoff from 5 to 4. We unearth evidence for a self-similarity of the triggering process as models trained on lower magnitude events have the same forecasting skills as models trained on higher magnitude earthquakes. Expressing our forecasts in terms of the full distribution of earthquake rates at different spatial resolutions, we present tests for the consistency of our model, which is often found satisfactory but also points to a number of potential improvements, such as incorporating anisotropic spatial kernels, and accounting for spatial and depth dependant variations of the ETAS parameters. The model has been implemented as a reference model on the global earthquake prediction platform RichterX, facilitating predictive skill assessment and allowing anyone to review its prospective performance.
Publications 1 - 2 of 2