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Figure 7. This section between 2247.5 and 2248.0 m shows the comparison between stratigraphy and LLS much deeper in the core. It can be seen that the methodology used from the top of the core still holds and the comparisons are excellent. In this case, there were 42 stratigraphic and 39 LLS annual layer markers identified.

Figure 7 This section between 2247.5 and 2248.0 m shows the comparison between stratigraphy and LLS much deeper in the core. It can be seen that the methodology used from the top of the core still holds and the comparisons are excellent. In this case, there were 42 stratigraphic and 39 LLS annual layer markers identified.

Related Figures (13)

Figure 1. Stratigraphic layering of a 1.5-m snow pit exca- vated near the GISP2 drilling site in August 1990. The summer and winter stratigraphic sequences are readily delineated. Summer stratigraphy is dominated by layers of depth hoar which were utilized as the primary marker for delineating an- nual layers in dating the top 50% of the ice sheet at GISP2.
Figure 1. Stratigraphic layering of a 1.5-m snow pit exca- vated near the GISP2 drilling site in August 1990. The summer and winter stratigraphic sequences are readily delineated. Summer stratigraphy is dominated by layers of depth hoar which were utilized as the primary marker for delineating an- nual layers in dating the top 50% of the ice sheet at GISP2.
Table 1. List of Volcanoes Used as Tie Points to Verify the Dating of the GISP2 Core Throughout Recorded History
Table 1. List of Volcanoes Used as Tie Points to Verify the Dating of the GISP2 Core Throughout Recorded History
S, stratigraphy; D, isotopes; E, ECM; L, LLS; V, volcanics. Table 2. Error Estimates of the GISP2 Depth-Age Scale
S, stratigraphy; D, isotopes; E, ECM; L, LLS; V, volcanics. Table 2. Error Estimates of the GISP2 Depth-Age Scale
Figure 2. Example of multiparameter comparisons between 100 and 104 m. Each parameter in this se- quence yielded 16 years of accumulation. Lags and leads between the parameters are evident and expected as timing of the seasonal inputs varies slightly between years and between parameters.
Figure 2. Example of multiparameter comparisons between 100 and 104 m. Each parameter in this se- quence yielded 16 years of accumulation. Lags and leads between the parameters are evident and expected as timing of the seasonal inputs varies slightly between years and between parameters.
Figure 3. Multiparameter sequence between 270 and 275 m. Twenty stratigraphic layers and 6'®O peaks were counted in conjunction with 22 ECM and 23 LLS peaks. Annual layer markers for LLS, ECM, and 68O are shown as vertical white lines. These correspond to the spring/summer inputs for each parameter. Details of the interpretation of this record are given in the text.
Figure 3. Multiparameter sequence between 270 and 275 m. Twenty stratigraphic layers and 6'®O peaks were counted in conjunction with 22 ECM and 23 LLS peaks. Annual layer markers for LLS, ECM, and 68O are shown as vertical white lines. These correspond to the spring/summer inputs for each parameter. Details of the interpretation of this record are given in the text.
Figure 4. In this 2-m section from 501 to 503 m, 11 stratigraphic summers were identified in conjunction with 12 ECM peaks and 11 LLS peaks. A final count of 11 annual layers was determined for this 2-m section of ice. The stratigraphic sequence corresponding to the ECM and LLS peaks at 501 m is actually in the 500-m section and was counted there. At approximately 502.72 m a stratigraphic sequence is present as well as a strong ECM signal; however, the LLS signal is very weak and was not counted, resulting in an undercount in the LLS record. However, this did not affect the final count. Also, at approximately 502.9 m a stratigraphic sequence was observed, but there is not a corresponding strong ECM or LLS peak and this sequence was thus dropped from the count. Again, this section of ice shows that although the actual depths of the peaks are slightly offset, approximately the same number of years is revealed for each parameter. section and constituted the primary annual layer indicator. However, scattered ECM and/or LLS data from the brittle ice zone were available, and they provided useful checks on strati- graphic layer counts. A comparison of the stratigraphic and LLS records is shown in Figure 5 between 871 and 873 m. Both parameters exhibit 12 peaks in this section, but because of lags or leads the final picks do not always coincide. It is evident that although the mechanical condition of the ice deteriorated somewhat in this region, the parameters used for layer identi- The brittle ice zone (719-1371 m) was a region of some concern due to the large number of fractures in the core [Gow et al., this issue]. While this fracturing of the ice core interfered particularly with the ECM and LLS measurements, visual stra- tigraphy remained very clear and readily decipherable in this
Figure 4. In this 2-m section from 501 to 503 m, 11 stratigraphic summers were identified in conjunction with 12 ECM peaks and 11 LLS peaks. A final count of 11 annual layers was determined for this 2-m section of ice. The stratigraphic sequence corresponding to the ECM and LLS peaks at 501 m is actually in the 500-m section and was counted there. At approximately 502.72 m a stratigraphic sequence is present as well as a strong ECM signal; however, the LLS signal is very weak and was not counted, resulting in an undercount in the LLS record. However, this did not affect the final count. Also, at approximately 502.9 m a stratigraphic sequence was observed, but there is not a corresponding strong ECM or LLS peak and this sequence was thus dropped from the count. Again, this section of ice shows that although the actual depths of the peaks are slightly offset, approximately the same number of years is revealed for each parameter. section and constituted the primary annual layer indicator. However, scattered ECM and/or LLS data from the brittle ice zone were available, and they provided useful checks on strati- graphic layer counts. A comparison of the stratigraphic and LLS records is shown in Figure 5 between 871 and 873 m. Both parameters exhibit 12 peaks in this section, but because of lags or leads the final picks do not always coincide. It is evident that although the mechanical condition of the ice deteriorated somewhat in this region, the parameters used for layer identi- The brittle ice zone (719-1371 m) was a region of some concern due to the large number of fractures in the core [Gow et al., this issue]. While this fracturing of the ice core interfered particularly with the ECM and LLS measurements, visual stra- tigraphy remained very clear and readily decipherable in this
Figure 5. This section from 871 to 873 m is from the brittle ice zone. In this zone, stratigraphy was the primary annual signal indicator and ECM and LLS were used when available. In this section, 12 annual layer markers are observed in both the stratigraphy and the LLS, indicating that counting is still viable within the brittle ice zone.
Figure 5. This section from 871 to 873 m is from the brittle ice zone. In this zone, stratigraphy was the primary annual signal indicator and ECM and LLS were used when available. In this section, 12 annual layer markers are observed in both the stratigraphy and the LLS, indicating that counting is still viable within the brittle ice zone.
Figure 6. Photograph of a 19-cm-long section of ice from 1855 m showing annual stratigraphic layers in the Wisconsinan. This section contains 12 summer layers (arrowed) sandwiched between darker winter layers. The lighter layers are a result of increased scattering of light due to higher concentrations of dust. As records were obtained, the actual depth recorded for Between 1370 and 1678 m, in the Preboreal preceeding the onset of the Younger Dryas, the character of the visual strati- graphic signal changed drastically from that of the character- istic depth-hoar sequences to one consisting of alternating clear and cloudy bands with the cloudy bands containing much higher concentrations of dust than were present in younger Holocene ice. The region between 1680 and 1800 m consists of the Younger Dryas, the Bglling/Allergd, and the transition to full glacial conditions. Once into the full glacial (Wisconsinan) period, below 1800 m, dust concentrations increased signifi- cantly and the magnitude of changes seen in the ECM record decreased. As a result of this and perhaps because of limita- tions imposed by the spatial resolution of the ECM technique, it became increasingly difficult to use the ECM record for annual layer signal identification in deeper ice-age ice. A pho- tograph of a 19-cm-long section of core in Figure 6 demon- strates the very distinct annual layering associated with 12 summer peaks, based on the bright white light bands corre- sponding to spring/summer dust rich layers in the core. A 0.5-m section of stratigraphic and LLS records from between
Figure 6. Photograph of a 19-cm-long section of ice from 1855 m showing annual stratigraphic layers in the Wisconsinan. This section contains 12 summer layers (arrowed) sandwiched between darker winter layers. The lighter layers are a result of increased scattering of light due to higher concentrations of dust. As records were obtained, the actual depth recorded for Between 1370 and 1678 m, in the Preboreal preceeding the onset of the Younger Dryas, the character of the visual strati- graphic signal changed drastically from that of the character- istic depth-hoar sequences to one consisting of alternating clear and cloudy bands with the cloudy bands containing much higher concentrations of dust than were present in younger Holocene ice. The region between 1680 and 1800 m consists of the Younger Dryas, the Bglling/Allergd, and the transition to full glacial conditions. Once into the full glacial (Wisconsinan) period, below 1800 m, dust concentrations increased signifi- cantly and the magnitude of changes seen in the ECM record decreased. As a result of this and perhaps because of limita- tions imposed by the spatial resolution of the ECM technique, it became increasingly difficult to use the ECM record for annual layer signal identification in deeper ice-age ice. A pho- tograph of a 19-cm-long section of core in Figure 6 demon- strates the very distinct annual layering associated with 12 summer peaks, based on the bright white light bands corre- sponding to spring/summer dust rich layers in the core. A 0.5-m section of stratigraphic and LLS records from between
Figure 8. A thin section photograph taken between crossed polarizers showing stratigraphy in a section between 2552.60 and 2552.80 compared to the LLS record. Most of the bright white layers (containing elevated levels of dust) compare closely to the peaks in the LLS record. It is believed that viable dating is retained to 2800 m below which flow disturbance in the ice becomes quite severe [Alley et Because of the above discrepancy the senior author returned to the National Ice Core Laboratory and rechecked the visible stratigraphy. No significant changes from the original counts were observed. Study of the LLS record, now available in a more detailed format using a 1-mm beam width rather than 8
Figure 8. A thin section photograph taken between crossed polarizers showing stratigraphy in a section between 2552.60 and 2552.80 compared to the LLS record. Most of the bright white layers (containing elevated levels of dust) compare closely to the peaks in the LLS record. It is believed that viable dating is retained to 2800 m below which flow disturbance in the ice becomes quite severe [Alley et Because of the above discrepancy the senior author returned to the National Ice Core Laboratory and rechecked the visible stratigraphy. No significant changes from the original counts were observed. Study of the LLS record, now available in a more detailed format using a 1-mm beam width rather than 8
Figure 10. Example of an LLS record from 2808 m. Some faint stratigraphic layering is decipherable in this meter of core and appears to be disturbed. Even though the stratigraphic record is disturbed in this section of core, the LLS record appears to be intact, and its overall character is identical to that observed in undisturbed ice at higher levels in the ice sheet.
Figure 10. Example of an LLS record from 2808 m. Some faint stratigraphic layering is decipherable in this meter of core and appears to be disturbed. Even though the stratigraphic record is disturbed in this section of core, the LLS record appears to be intact, and its overall character is identical to that observed in undisturbed ice at higher levels in the ice sheet.
Figure 11. An example of the LLS record from ice that is very clear and coarse grained with little visible stratigraphy.
Figure 11. An example of the LLS record from ice that is very clear and coarse grained with little visible stratigraphy.
Figure 12. An example of the LLS record from ice that contains decipherable stratigraphic layering and i: much finer grained than the section of ice presented in Figure 11. Putting error estimates on the depth-age scale is difficult. The accuracy of the depth-age scale can be checked against historically dated volcanic events over the last 2000 years (i.e., tie points; Table 1). If a discrepancy existed between the date of a known volcanic eruption and the corresponding date as- signed on the basis of elevated ECM and sulfate peaks, the strengths of the various annual signals were then reevaluated to determine if perhaps a more accurate date could be ob- tained. Most of our final dates of the major historical volcanic eruptions identified in the core match stratigraphically defined dates precisely. To obtain an estimate of error based on the Assessing the accuracy of our depth-age scale based on the multiparameter approach in deeper ice becomes increasingly more difficult as there are no independent means of dating the ice by radiometric techniques. Possible comparisons might be made with events that have been dated in corals and deep-sea cores, but these events likely have larger errors than those applying to the GISP2 timescale. Additionally, a depth-age scale exists for the European GRIP core (largely based on flow modeling and noncontinuous methods) which can also be used for comparison. The errors that are listed in Table 2 are based
Figure 12. An example of the LLS record from ice that contains decipherable stratigraphic layering and i: much finer grained than the section of ice presented in Figure 11. Putting error estimates on the depth-age scale is difficult. The accuracy of the depth-age scale can be checked against historically dated volcanic events over the last 2000 years (i.e., tie points; Table 1). If a discrepancy existed between the date of a known volcanic eruption and the corresponding date as- signed on the basis of elevated ECM and sulfate peaks, the strengths of the various annual signals were then reevaluated to determine if perhaps a more accurate date could be ob- tained. Most of our final dates of the major historical volcanic eruptions identified in the core match stratigraphically defined dates precisely. To obtain an estimate of error based on the Assessing the accuracy of our depth-age scale based on the multiparameter approach in deeper ice becomes increasingly more difficult as there are no independent means of dating the ice by radiometric techniques. Possible comparisons might be made with events that have been dated in corals and deep-sea cores, but these events likely have larger errors than those applying to the GISP2 timescale. Additionally, a depth-age scale exists for the European GRIP core (largely based on flow modeling and noncontinuous methods) which can also be used for comparison. The errors that are listed in Table 2 are based
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