Correlation of Marine ¹⁴C Ages from the Nordic Seas with the GISP2 Isotope Record: Implications for ¹⁴C Calibration Beyond 25 ka BP

Abstract

ABSTRACT. We present two new high-resolution sediment records from the southwestern Iceland and Norwegian Seas that were dated by numerous 14C ages up to 5414C ka BP. Based on various lines of evidence, the local 14C reservoir effect was restricted to 400-1600 yr. The planktic stable isotope records reveal several meltwater spikes that were sampled with an average time resolution of 50 yr in PS2644 and 130 yr in core 23071 during isotope stage 3. Most of the 6180 spikes correlate peak-by-peak to the stadials and cold rebounds of the Dansgaard-Oeschger cycles in the annual-layer counted GISP2 ice core, with the major spikes reflecting the Heinrich events 1-6. This correlation indicates large fluctuations in the calibration of 14C ages between 20 and 54 i4C ka BP. Generally the results confirm the 14C age shifts as predicted by the geomagnetic model of Laj, Mazaud and Duplessy (1996). However, the amplitude and speed of the abrupt decrease and subsequent major increase of our 14C shifts after 4514C ka BP clearly exceed the geomagnetic prediction near 40-43 and 32-34 calendar (cal) ka BP. At these times, the geomagnetic field intensity minima linked to the Laschamp and the Mono Lake excursions and confirmed by a local geomagnetic record, probably led to a sudden increase in cosmogenic 14C and 10Be production, giving rise to excess 14C in the atmosphere of up to 1200%o.

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“…These anomalies differ from the 'constant' age shift of 3500 years, originally inferred from three first coral 14 C dates calibrated by U-Th ages (Winn et al, 1991;Bard et al, 1998). 14 C-age shifts that exceed 3500 years, despite a potentially high 14 C-reservoir effect, imply a substantially increased rate of radiocarbon production during that time, similar to rates found in the Nordic Seas (Völker et al, 1998).…”

“…If this age also holds true for the (upwelled) estuarine surface water of the glacial SCS, the resulting 14 C anomaly was just opposed to the age difference resulting from an increased cosmogenic 14 C production in the atmosphere during that time, that led to a reduction of 14 C ages by 3000 to 4000 years versus calendar ages (Bard et al, 1990a;Völker et al, 1998).…”

“…For the calculation of accumulation rates we converted the 14 C ages into calendar ages, which implied two major unknowns: (1) changes in the 14 C-reservoir effect of the surface water in the SCS; and (2) the differential 14 C-production rates in the atmosphere during the last glacial cycle (Duplessy et al, 1989;Stuiver and Braziunas, 1993;Laj et al, 1996;Völker et al, 1998). The 14 C ages of dissolved CO 2 in surface-water samples from the SCS (unpublished data) show a post-bomb age of 750 years.…”

“…In δ 18 O Stage 3 the conversion of 14 C dates into calendar ages is more complicated than in Stages 1-2 because of (1) poorly quantified but strong variations in cosmogenic 14 C production (Laj et al, 1996;Völker et al, 1998), which result in (2) various 14 C rises and plateaus that lack any U=Th-calibrated age-control points (Bard et al, 1990a), and finally, (3) because of unknown but probably considerable variations in the local 14 C-reservoir effect. Therefore, we adopted a different, 'direct' approach to chronostratigraphy: we tied the δ 18 O minima of core 17940 in Stages 2 and 3 to ice-varve ages of the warm D-Oe events recorded in the δ 18 O curve of the GISP2 ice core (Grootes and Stuiver, 1997).…”

“…Error bars show 1¦ standard error of 14 C dates. Thin lines depict global 14 C-age shift (visual-fit trend line and envelope) estimated from 14 C values measured in two Nordic Sea cores and tuned to the GISP2 ice record (Völker et al, 1998). Differences in 14 C shifts between records from SCS and Nordic Seas are assigned to differential 14 C-reservoir effect of SCS surface water.…”