Identifying sediment discontinuities and solving dating puzzles using monitoring and palaeolimnological records

Abstract

Palaeolimnological studies should ideally be based upon continuous, undisturbed sediment sequences with reliable chronologies. However for some lake cores, these conditions are not met and palaeolimnologists are often faced with dating puzzles caused by sediment disturbances in the past. This study chooses Esthwaite Water from England to illustrate how to identify sedimentation discontinuities in lake cores and how chronologies can be established for imperfect cores by correlation of key sediment signatures in parallel core records and with longterm monitoring data . Replicated short cores (ESTH1, ESTH7, and ESTH8) were collected and subjected to loss-on-ignition, radiometric dating ( 210 Pb, 137 Cs, and 14 C), particle size, trace metal, and fossil diatom analysis. Both a slumping and a hiatus event were detected in ESTH7 based on comparisons made between the cores and the long-term diatom data. Ordination analysis suggested that the slumped material in ESTH7 originated from sediment deposited around 1805-1880 AD. Further, it was inferred that the hiatus resulted in a loss of sediment deposited from 1870 to 1970 AD. Given the existence of three superior 14 C dates in ESTH7, ESTH1 and ESTH7 were temporally correlated by multiple palaeolimnological proxies for age-depth model development. High variability in sedimentation rates was evident, but good agreement across the various palaeolimnological proxies indicated coherence in sediment processes within the coring area. Differences in sedimentation rates most likely resulted from the natural morphology of the lake basin. Our study suggests that caution is required in selecting suitable coring sites for palaeolimnological studies of small, relatively deep lakes and that proximity to steep slopes should be avoided wherever possible. Nevertheless, in some cases, comparisons between a range of contemporary and palaeolimnological records can be employed to diagnose sediment disturbances and establish a chronology.

Highly variable sediment deposition in Lake Imandra, NW Russia, since the Late Pleistocene

“…Prominent shifts in the sediment colour, Ti/Al ratios, TOC/TN, TOC and water content between SU Ib and SU Ie in core Co1411 may be associated with a hiatus (Fig. 11; Dong et al ., 2016). In core Co1410, there is a more gradual colour change from brownish to greyish sediment between SU Ib and SU Ic, and the greyish sediments of SU Ic have different characteristics.…”

Deciphering long‐term records of natural variability and human impact as recorded in lake sediments: a palaeolimnological puzzle

“…The ideal palaeolimnological study is conducted on continuous, undisturbed sediment sequences with reliable chronologies. However for many lake cores, these conditions are not necessarily met, where the cores are exposed to issues related to mixing, sediment focusing (see Box ) or human disturbance of the record (dredging for example) . A well‐designed study, with careful site selection can often negate some of these effects .…”

“…However for many lake cores, these conditions are not necessarily met, where the cores are exposed to issues related to mixing, sediment focusing (see Box 2) or human disturbance of the record (dredging for example). 173,174 A well-designed study, with careful site selection can often negate some of these effects. 175 What is desirable, however, is the use of a multiproxy, multidisciplinary, multi-site approach to analyzing sediment records, allowing some of the ambiguity regarding the interpretation of the records to be circumnavigated.…”