An 83 000 year old ice core from Roosevelt Island, Ross Sea, Antarctica

Abstract

Abstract. In 2013, an ice core was recovered from Roosevelt Island in the Ross Sea, Antarctica, as part of the Roosevelt Island Climate Evolution (RICE) project. Roosevelt Island is located between two submarine troughs carved by paleo-ice-streams.The RICE ice core provides new important information about the past configuration of the West Antarctic Ice Sheet and its retreat during the most recent deglaciation. In this work, we present the RICE17 chronology and discuss preliminary observations from the new records of methane, the isotopic composition of atmospheric molecular oxygen (δ 18O atm ), the isotopic 5 composition of atmospheric molecular nitrogen (δ 15 N-N 2 ), and total air content (TAC). RICE17 is a composite chronology combining annual layer interpretations, gas synchronization, and firn modeling strategies in different sections of the core. An automated matching algorithm is developed for synchronizing the high-resolution section of the RICE gas records (60-720 m, 1971 CE to 30 ka) to corresponding records from the WAIS Divide ice core, while deeper sections are manually matched. Ice age for the top 343 m (2635 yr BP, before 1950 C.E.) is derived from annual layer interpretations and described in the accom-10 panying paper by Winstrup et al. (2017). For deeper sections, the RICE17 ice age scale is based on the gas age constraints and the ice age-gas age offset estimated by a firn densification model. Novel aspects of this work include: 1) stratigraphic matching of centennial-scale variations in methane for pre-anthropogenic time periods, a strategy which will be applicable for developing precise chronologies for future ice cores, 2) the observation of centennial-scale variability in methane throughout the Holocene which suggests that similar variations during the late pre-15 industrial period need not be anthropogenic, and 3) the observation of continuous climate records dating back to ∼65 ka which provide evidence that the Roosevelt Island Ice Dome was a constant feature throughout the last glacial period.

The SP19 chronology for the South Pole Ice Core - Part 2: gas chronology, Δage, and smoothing of atmospheric records

“…The SPC14 record validates previous observations of persistent centennial-scale CH4 variability through the Holocene and in the glacial period (Mitchell et al, 2013;Rhodes et al, 2017;Lee et al, 2018) including variations matched with the WD CH4 record back to 16,150 ka, just after the onset of the glacial termination (Fig. 7).…”

“…CH4 concentrations measured at OSU were made using a wet-extraction technique as described in Grachev et al (2009), with updates by Mitchell et al (2011) and Lee et al (2018). Briefly, subsamples of the main core measuring 10 cm x 6 cm x 2.5 10 cm (with the 10 cm dimension oriented parallel to the vertical axis of the core) were split into replicate samples by cutting along the vertical axis.…”

“…We used the WD discrete CH4 record for 0 -9.8 ka BP, and the continuous CH4 record for 9.8 ka BP until 54 ka BP (Rhodes et al, 2015;WAIS Divide Project Members, 2015). The WD2014 chronology has also been used with success for synchronization of other Antarctic ice cores 5 Lee et al, 2018). The resulting SPC14 CH4 record has an age resolution of 25 to 150 years, which is sufficient for resolving all of the major abrupt CH4 variations of the last 54,000 years as well as smaller-scale Holocene variations.…”

“…The gas and ice chronologies are collectively referred to as the SP19 chronology. CH4 is a well-mixed atmospheric trace gas exhibiting globally synchronous abrupt variations on decadal to millennial scales (Blunier and Brook, 2001;Brook et al, 1996;Lee et al, 2018;Rhodes et al, 2015), making it an ideal choice 25 for stratigraphic matching to existing ice core records. The chronology presented here relies on correlating CH4 variations between the SPC14 and the West Antarctic Ice Sheet Divide (WD) ice cores, using millennial-scale abrupt variations during the last ice age and glacial-interglacial transition, and centennial-scale variations during the Holocene.…”

Authors' response to RC#1-3

“…The interpretation and identification of key events in the climate history thus depend on accurate depth alignment. This is particularly important deeper in the core, where a misalignment of a few centimetres could equate to hundreds or even thousands of years (Lee et al, 2018). Alignment of the isotope data to the depth scale is based on the time lag between the depth log file and the WVIA instrument output.…”