Holocene evolution of seasonal stratification in the Celtic Sea: refined age model, mixing depths and foraminiferal stratigraphy

摘要

Published stable isotopic (oxygen, carbon) and preliminary foraminiferal data from a Holocene vibrocore from the Celtic Sea (Northwest European continental shelf) have been interpreted in terms of the progressive replacement of tidally mixed by seasonally stratified water, the first study of the long-term dynamics of seasonal stratification. This study was hampered by poor age control and the foraminiferal data were based on processing with a 125-mu m sieve which has been shown not to recover critical small taxa. We present here a new age model for this vibrocore (BGS 51/-07/199) based on 12 accelerator mass spectrometry ~(14)C dates on foraminifera and molluscs, radionuclide (~(137)Cs, ~(40)K) profiles from the vibrocore and juxtaposed multicores, and a complete foraminiferal stratigraphy based on sieving at 63 mu m. Together, the three datasets enable a mixing model to be proposed which clarifies the resolution of the record which is confirmed as extending from the Late-Glacial to the late Holocene. Correction for autocompaction reveals an increase in sedimentation rate and mixing depth from the early to the late Holocene. A temporary increase in sedimentation rate at 6650 yr cal BP is attributed to migration of the productive frontal zone across the core site. Some time after 3000 yr cal BP sedimentation either slowed abruptly or ceased completely, giving the modern mixed layer an apparent age of approx 3000 yr cal BP. The mixed layer depth indicates strongly that the apparent transition to stratification during the early Holocene is primarily a threshold change attenuated by bioturbation; secondary attentuation is related to reworking. The mixed layer model adopted suggests that this isotopic transition occured between 8990 and 8440 yr cal BP (8720 +- 2 sigma). The foraminiferal analyses, in the light of modern foraminiferal distributional data, support the inference that the succession can be interpreted as a response to the progressive seasonal stratification of the Celtic Sea during the Holocene. The data highlight the value of key taxa as indicators of shelf palaeostratification in the geological record, notably of Testularia bocki and Stainforthia fustformis as mixed-frontal and frontal-stratified indicators, respectively. The critical change to this frontal assemblage occurs at the same depth as the isotopic threshold, based on the mixing model. These data affirm the earlier interpretation of the isotopic record as registering bottom water temperature and water column productivity changes driven by the evolution of seasonal stratification. The clear association between grain size and sedimentation, as well as these isotopic and faunal data, indicates that the shelf record of highstand sedimentation is preferentially biased towards the preservation of sequences deposited under seasonally stratified rather than mixed water masses.