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Chemical and morphological features of spores and pollens have been linked tochanges in solar ultraviolet radiation (specifically UVB, 280-315 nm) atEarth's surface. Variation in UVB exposure as inferred from these features hasbeen suggested as a proxy for paleoaltitude. While UVB irradiance does increasewith altitude above sea level, a number of other factors affect the irradianceat any given place and time. In this modeling study we use the TUV atmosphericradiative transfer model to investigate dependence of surface-level UVBirradiance and relative biological impact on a number of constituents inEarth's atmosphere that are variable over long and short time periods. Weconsider changes in O3 column density, and SO2 and sulfate aerosols due toperiods of volcanic activity, including that associated with the formation ofthe Siberian Traps. We find that UVB irradiance may be highly variable undervolcanic conditions and variations in several of these atmospheric constituentscan easily mimic or overwhelm changes in UVB irradiance due to changes inaltitude. On the other hand, we find that relative change with altitude is notvery sensitive to different sets of atmospheric conditions. Any paleoaltitudeproxy based on UVB exposure requires confidence that the samples undercomparison were located at roughly the same latitude, under very similar O3 andSO2 columns, with similar atmospheric aerosol conditions. In general, accurateestimates of the surface-level UVB exposure at any time and location requiredetailed radiative transfer modeling taking into account a number ofatmospheric factors; this result is important for paleoaltitude proxies as wellas attempts to reconstruct the UV environment through geologic time and to tieextinctions, such as the end-Permian mass extinction, to UVB irradiancechanges.

earth and planetary science letters

Atmospheric constituents and surface-level UVB: implications for a  paleoaltimetry proxy and attempts to reconstruct UV exposure during volcanic  episodes