D4i Greenland ice-core non-sea-salt sulfur concentrations and calculated volcanic sulfate deposition (1733-1900 CE)

The D4 ice core was recovered in 2003 from a high elevation, cold site in the dry snow region of the Greenland ice sheet (71.4°N, 43.9°W, 2730 m asl, 41 cm water equivalent per year annual accumulation rate). Initial collection and previous analyses of the core have been reported earlier (e.g., McConnell et al., 2007). New longitudinal samples of ~32 by ~32 mm were cut (D4i) from contiguous sections of the remaining D4 core archived in a commercial storage facility in Reno. The samples from near and below the pore close-off depth (66-145 m depth; dating 1733-1900 CE) were analyzed continuously using a state-of-the-art ice core analytical system. https://icecores.org/inventory/d4 . Mean monthly D4i non-sea-salt sulfur concentrations corrected for sea-salt sulfur contribution based on [Na], assuming a S/Na mass ratio of seawater of 0.084 [Bowen, 1979]. Estimated mean monthly D4i median pre-industrial (1733-1875 CE) sulfur background concentrations after excluding volcanic events identified as monthly values exceeding the median concentrations plus two times the interquartile range (Q3 minus Q1). D4i Excess-sulfur concentrations defined as non-sea-salt sulfur concentrations minus sulfur background concentrations; negative values set to 0; using monthly resolved excess-sulfur, we estimated long-term trends with a 61-yr running median (RM); we attributed a volcanic origin to values exceeding a threshold of the RM + 4 x MAD (median of absolute deviation); we multiplied each monthly excess-sulfur concentration value exceeding the detection treshold with 0.41 water equivalent per year / 12 (i.e., assuming constant snowfall rate throughout the year) to derive monthly volcanic sulfate deposition at the D4 site.