Climate Effects on Firn Microstructure are Preserved within the Firn Column
Abstract
Interpretation of ice-core paleoclimate records requires an understanding of the transition process from snow to firn to glacial ice. This process, termed densification, typically is modeled using temporally broad-scale estimates of temperature and accumulation rate. However, it has been shown that short-term climate events permanently alter firn microstructure. This altered microstructure may impact the depth of pore close-off and gas flow through firn, potentially leading to inaccurate estimates of gas age-ice age difference (Δage). Here, we further investigate impacts of short-term climatic events on firn microstructure evolution by examining physical properties of firn at the NEEM site in Greenland. Measurements of the firn included density, permeability, and structural parameters measured from x-ray micro-computed tomography. Our results indicate that firn layers found at 24 to 29 m depth display expected density but unexpected permeability values, indicating a shift in firn microstructure during the years 1935 to 1952. Accumulation rates and δ18O from a compilation of nearby shallow cores indicate that accumulation remained stable during these time periods, while annual average temperatures were high in 1928 and the late 1930s. Combined with other temperature records, our results indicate that the sharp rise in Arctic temperature during the late 1930s and early 1940s caused metamorphism of the near-surface firn, which was preserved in the microstructure at depth. Importantly, average density remained undisturbed throughout this warm period. Our results show that climate fluctuations, including changes in temperature without changes in accumulation rate, permanently change firn microstructure. Without taking these changes into consideration, estimates of Δage will be inaccurate.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.C41C1748K
- Keywords:
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- 3337 Global climate models;
- ATMOSPHERIC PROCESSESDE: 0724 Ice cores;
- CRYOSPHEREDE: 1621 Cryospheric change;
- GLOBAL CHANGEDE: 4901 Abrupt/rapid climate change;
- PALEOCEANOGRAPHY