DOI: 10.1594/pangaea.943194
title: Greenland NGRIP2 Ice-Core annual layer markings

description: Annual markers forming the GICC05 time scale for NGRIP1, NGRIP2, GRIP, and DYE-3 where data was available and sufficiently resolved to allow annual dating. The markers are placed in the winter and spring depending on the availability of data (e.g. using the winter d18O minimum, winter Sodium concentration maximum, spring dust/Calcium concentration maximum, or visual stratigraphy grey-scale peaks in the deepest parts). Across data gaps, markers are placed by interpolation or using other impurity species with different seasonality (e.g. using summer Ammonium or Nitrate peaks). Therefore, the criteria for where the annual markers are places vary between sections, and care should be taken when interpreting data on annual scale. Ages are reported as years before A.D. 2000 / 2000 CE, abbreviated b2k. Depths (in meter) are true depths below the undisturbed surface the year when drilling started.
description: Greenland Ice-Core Chronology 2005 (GICC05) annual layer depths for various Greenland ice cores. This is the high-resolution version (full, annual resolution) data file. Previously, 10- and 20-year resolution data files containing the time scale and resampled d18O data have been released for different time intervals together with the papers mentioned below. Ages are reported as years before A.D. 2000 / 2000 CE, abbreviated b2k.The file contains the location of the annual markers in the GICC05 time scale for each core's depth sections where data was available and sufficiently resolved to allow annual dating. Details are given in the papers listed below. The markers are placed in the winter and spring depending on the availability of data (e.g. using the winter d18O minimum, winter Sodium concentration maximum, spring dust/Calcium concentration maximum, or visual stratigraphy grey-scale peaks in the deepest parts). Across data gaps, markers are placed by interpolation or using other impurity species with different seasonality (e.g. using summer Ammonium or Nitrate peaks). Therefore, the criteria for where the annual markers are places vary between sections, and care should be taken when interpreting data on annual scale. The dating of the 0-7.9 ka b2k part is described in the paper Vinther et al., 2006The dating of the 7.9-14.7 ka b2k part is described in the paper Rasmussen et al., 2006The dating of the 14.7-41.8 ka b2k part is described in the paper Andersen et al., 2006The dating of the 41.8-60.0 ka b2k part is described in the paper Svensson et al., 2008When counting layers, uncertainty is introduced when an annual layer is backed up by evidence only in some of the data series, or when a certain well-resolved feature is suspected to contain more than one annual layer. The cases of ambiguity in the annual layer identification process have been marked using so-called uncertain layer markings. These uncertain layer markings were included in the time scale as ½ ± ½ years, with the ± ½ years forming the basis for quantifying the so-called maximum counting error. The concept of maximum counting error is further discussed in Rasmussen et al. (2006). In a standard deviation context, the maximum counting error can be regarded as 2 sigma as discussed in Andersen et al. (2006).In the Holocene, GS-1, and GI-2, the published time scale was derived from annual layer markings by manually determining which half of the uncertain layer markings to count as years, and which to skip. The maximum counting error was estimated from the number of uncertain layer markings as a constant relative uncertainty for each period with similar data availability and characteristics: 21-3,845 a b2k (0.25%), 3,846-6,905 a b2k (0.5%), 6,906-10,276 a b2k (2%), 10,277-11,703 a b2k (0.67%), 11,703-12,896 a b2k (3,3%), 12,896-14,075 a b2k (2.6%), 14,075-14,692 a b2k (2.7%) (see table 2 in Vinther et al, 2006, and table 3 in Rasmussen et al., 2006). From GS-2 and below (Andersen et al., 2006; Svensson et al., 2008) every 2nd uncertain layer was counted as a year and the maximum counting uncertainty increased by one year (giving rise to a variable relative counting error ranging from 4% in the warm interstadial periods to 7% in the cold stadials, and averaging 5.3%). In data set "Greenland NGRIP2 Ice-Core annual layer markings"(https://doi.pangaea.de/10.1594/PANGAEA.943194), the depths of the annual layer markings (including the uncertain ones) are provided with indication of which of these were counted as annual layers. This data set is only available below 10.2 ka. Above this, the locations of the discarded half of the uncertain layer markings have only been recorded on paper.The NGRIP1 core reaches down to a depth of 1372 m. The NGRIP2 core (drilled 20 meters away from the NGRIP1 core) reaches from surface to bedrock, but high-resolution measurements have only been carried out downwards from 1346 m. In the 26 m overlap zone, the cores are offset by 0.43 m on average, probably due to uncertainties in the logging procedure (see Schøtt Hvidberg et al., Ann. Glac. 35, 2002). Thus, the same features appear at larger depths in the NGRIP1 than in the NGRIP2 core. We recommend that NGRIP1 depths are used until 9820 b2k, and NGRIP2 depths are used below this.Note that the GICC05 time scale has later been revised. The first section of the new time scale, named GICC21, is described in the paper "A multi-ice-core, annual-layer-counted Greenland ice-core chronology for the last 3800 years: GICC21", Climate of the Past volume 18, p. 1125-1150, 2022, https://doi.org/10.5194/cp-18-1125-2022. Updated GICC21 annual-layer positions are released in the supplement to the paper.