An Evaluation of Antarctic Ice Core Nitrate Records as a Proxy for Solar Activity

Nitrate (NO₃⁻) deposition in polar ice sheets archives valuable information on past solar activity. However, interpretation of Antarctic ice core NO₃⁻ records as a proxy for past solar activity remains challenging due to multiple sources and processes controlling NO₃⁻ variability in ice core records. Here, we present a new high-resolution ice core NO₃⁻ record (1905–2005 CE) from coastal Dronning Maud Land, East Antarctica, to investigate the solar signal and other forcing factors/processes in controlling ice core NO₃⁻ variability. Our record exhibits significant periodicity in the range of 8–12 years frequency band during 1940–2005 CE, apparently identified as the signal of ∼11 year sunspot cycle; however, such signal was not detected in the previous interval during 1905–1940 CE. To address the discontinuous and/or obscured signals in the present ice core record and inconsistency among various Antarctica ice core records, we extended our investigations to 10 ice core NO₃⁻ records from various regions of Antarctica. Analysis of seven records for the common interval from 1738 to 1990 CE reveals dominant periodicities of 8–12 years, indicating solar forcing as a primary driver, followed by precipitation modulated by El Niño-Southern Oscillation and Pacific Decadal Oscillation. Further, our investigation reveals that the solar signal extracted from multiple records becomes undetectable when mean annual hemispheric sunspot numbers larger than 140, suggesting this is a threshold limit for detecting the solar signal. These findings will improve our present understanding of ice core NO₃⁻ records as a proxy for past solar activity.