One of the primary advantages of the event stratigraphy approach, is the ability to assess the representative nature of an event and its geographic expression. These are both fundamental questions that should be considered within palaeoenvironmental research. Replication of records, within the strictest sense, is limited within palaeoceanography; building local or basin wide event stratigraphies is one means of addressing this difficulty.
Replication within palaeoceanography is very limited but applying an event stratigraphy may enable assessment of how representative a feature is locally (e.g. turbidites), regionally (e.g. the D/O warming events of the last glacial, or more localised SST changes) and indeed on an ocean basin scale (e.g. Heinrich events). In addition, it can also enable the geographic expression of a feature to be determined, as demonstrated by the GS3 example where the NEA-GS3b warming event is seen only within the N.E. Atlantic. These two features of the event stratigraphies allow a local or regional event history to be constructed, even in the absence of secure age control, although a robust age model greatly facilitates comparisons between
An Evaluation of the Event Stratigraphy Approach in Marine Sediments palaeoenvironmental archives as well as enabling the duration of events to be estimated.
Where age control is available (e.g. the benthic δ18
O stacks or the Greenland ice core chronologies), perhaps the most valuable function of an event stratigraphy approach is the ability to assign an age from an event without the need to directly date that event within each record. However, this is strongly dependent upon a well constrained and robust chronology. Radiocarbon has been suggested as a means of obtaining an independent age model for palaeoenvironmental records but this method is often fraught with difficulties within the marine environment. Tuning to the Greenland ice cores does, however, provide well constrained age estimates for events within marine sediments that extend beyond the scope of radiocarbon. However, assumptions of synchronicity and linear interpolation as well as dating uncertainties should be remembered in interpreting either radiocarbon or transferred ice core chronologies. The assumptions implicit within both the application of an event stratigraphy (e.g. that all events are recorded in all records and that these events are synchronous) and the transferral of an age model from an event stratigraphy to an individual records (e.g. linear sedimentation between tie-points) remain problematic and require further testing, for example, further use of tephra isochrones may provide insights into the degree of synchronicity of events within different archives.
Event stratigraphies are invaluable in palaeoclimatic research, facilitating both age control and enabling the construction of local or regional event histories. The GS3 example demonstrates both of these. Whilst the INTIMATE protocol provides a sound means for correlation of events, as well as the assessment of any leads or lags in the climate system, it has proved difficult to obtain a sufficiently robust independent age model for the marine sediment sequences. Since the NEA-GS3b
event is not evident with the Greenland ice cores δ18
O record the timing of the event between the two archives cannot be compared. The INTIMATE protocol holds some promise for the elucidation of the phasing of palaeoclimatic events that are currently assumed to be synchronous. In practise, the application to marine sediment sequences is however, hampered by limitations of the radiocarbon method (e.g. low foraminiferal abundances, unknown variations in the local marine reservoir effect). However, by employing a form of event stratigraphy (but not that of the INTIMATE protocol), a regional feature not previously identified with the Greenland ice cores, has been documented for the NE Atlantic.
Correction of core ‘over-sampling’: An example using piston and gravity coring CHAPTER 5:
The correction of core ‘over-sampling’: An example using giant piston core MD04-2822 and gravity core +56-12/15CS from the Rockall Trough.
For palaeoceanographic studies, a primary aim during coring is the maintenance of stratigraphic and dimensional integrity of sediments in order to investigate depositional processes. The preservation of this stratigraphic ‘truth’ is essential when investigating depositional processes; sedimentation rates are often, although not exclusively, derived from the core dimensions (i.e. length of sediment recovered within a given timeframe). In practise, dimensional integrity is very difficult to achieve, due to the inherent problems of retrieving sediments from the deep ocean. One of the most pernicious effects of imperfect retrieval of sediments is over- or under-sampling, typically associated with the upper portions (<10 m) of piston and lower sections (>3 to 4 m) of gravity cores respectively. This distortion of sediment dimensions may lead to erroneous calculations of sediment fluxes. The effects of such over- or under-sampling may be very subtle so as to be un-noticeable during ordinary logging procedures; or extreme, instances of collapsed core liners and mid-core flow- in are reported (e.g. Bouma and Boerma 1968, McCoy 1985). The term over-sampling is used in preference to stretching as the latter would be associated with anomalously high porosities i.e. low densities and would require pore-water displacement (Széréméta et al 2004, Skinner and McCave 2003).
1. Background
Only a brief overview of coring techniques is offered here; for a full explanation, the reader is directed to the excellent review of Skinner and McCave (2003).
Over-sampling in piston cores is a result of movement within the core barrel of the piston, most likely due to cable recoil (McCoy 1985, Buckley 1994). As the cable accelerates upwards with rapid unloading at triggering, it pulls the piston with it. This creates a very low pressure within the core barrel as it moves downwards through the sediments. The potential extent of such piston recoil is determined by the water depth at the coring location and the whole weight of the coring equipment (Skinner and McCave 2003).
As an open barrel corer (piston or gravity) descends through sediments, an increasing (downward) vertical friction is exerted on the sediments below it (due to increasing material in the barrel and general increase in undrained shear strength of sediments with depth). With such piston movement, if the resulting negative pressure anomaly inside the core barrel (with respect to the sea floor pressure), is greater than the vertical friction stress, then a net upward (i.e. negative) pressure results below the descending corer. This exerts suction on the sediments (‘syringe effect’) leading to thickened or over-sampled sediments. The drop in pressure evolves as a function of time and develops independently of the downward friction stress; the sum of the opposing pressures varies throughout penetration leading to different down core effects (Skinner and McCave 2003) (Figure 5.1).
Based upon modelling, utilising principles of soil mechanics, Skinner and McCave (2003) demonstrated that imperfect piston coring does tend towards ‘ideal’ behaviour