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Stable isotope and foraminfera biofacies-analyses of the Middle Eocene to Oligocene successions in the southern North Sea Basin. Tools for stratigraphy and for reconstructions of the Oligocene extreme climates.
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The shallow marine successions from the
southern North Sea Basin serve as natural settings to investigate the
direct coupling between stratigraphy, eustasy and climate changes.
Benthic foraminifera are employed as a stratigraphic and
paleoenvironmental tool, and hence enable the construction of an
integrated sequence stratigraphic frame. They furthermore provide an
ideal geochemical record, which allows reconstructing the first high
resolution oxygen isotope curve from the southern North Sea Basin
covering the Middle Eocene to Upper Oligocene.
Although regional biostratigraphy achieves high resolution for the Cenozoic North Sea
Basin successions, calibration to the international time scale remained
problematic due to its semi-enclosed marginal marine setting, the
absence of age indicative calcareous
microfossils and a weak paleomagnetic signal. Resolving the
stratigraphic context of the North Sea Basin is particularly important,
since it is home of several historical unit-stratotype sections, some
of which are still debated among the international stratigraphic
community. This study contributes to the further elaboration of the
stratigraphy by the construction of a regional benthic foraminiferal
zonation scheme and tying it to the international time scale by means
of other biostratigraphies (dinocyst and nannoplankton), geochemical
dating techniques such as Sr-isotope stratigraphy, K-Ar dating and
cyclostratigraphy. The upper part of the Rupelian stratotype section is
documented in great detail, offering the opportunity to give
indications for future ratification of an international reference point
for the Rupelian/Chattian Stage boundary. Twelve benthic foraminiferal
biozones are newly defined (OO to OIX), covering the entire Oligocene.
Some species, such as Cassidulina
carapitana, Hoeglundina elegans, Turrilina alsatica, Rotaliatina bulimoides, Cibicidoides ungerianus, Rolfina arnei and Asterigerinoides guerichi,
play an important role for regional correlation, since comparison with
other onshore and offshore sections learns that they have a synchronous
range all over the North Sea Basin. Calibration to the international
magnetobiochronologic time scale is supported by dinocysts and
calcareous nannoplankton, often studied from the same samples.
This study upholds previous findings that the Eocene-Oligocene GSSP
level coincides with the base of the Bassevelde 3 Member. Benthic
foraminifera do not show any distinct bio-events at this level and the
biggest faunal and floral turnover occurs higher up-section, in between
the Watervliet Clay and Ruisbroek Sands. This level is
interpreted as the sequence boundary between the NS-Ru1/NS-Ru2
sequences, correlated to the ‘Grande Coupure’ which is associated with
the unconformity
between the more proximal Sint-Huibrechts Hern and Borgloon Formation. Within
the Ruisbroek Member, several benthic foraminifer species first appear, including
Hoeglundina elegans and Turrilina alsatica, and a little higher up-section, the marker species Cassidulina carapitana. The major bloom of Asterigerinoides guerichi allows
identification and correlation of the basal Chattian deposits
throughout the North Sea Basin. The major dinocyst events associated
with the Rupelian-Chattian transition in the North Sea Basin are the
first occurrence (FO) of Artemisiocysta cladodichotoma and the occurrence of Arctic dinocyst taxon Svalbardella spp. just above it. The
latter suggests a close link between the hiatus recorded in between the
Rupelian and Chattian historical stratotype sections, a global sea
level fall and an episode of profound cooling (Van Simaeys et al.,
2005a). The last occurrence (LO) of planktonic foraminifera Chiloguembelina
spp. occurs within the lower Rupelian in the southern North Sea Basin;
hence this event cannot be maintained as a biostratigraphic marker
event for the Rupelian/Chattian boundary in the international zonation
schemes.
Calibration of the southern North Sea biostratigraphic framework to the
international time scale largely depends on calcareous nannoplankton
biostratigraphy for the Lutetian Lede and Maldegem Formation
(Steurbaut, 1986, 1992; Vandenberghe et al., 2003; Steurbaut, 2006) and
on dinoflagellate cysts for the Bartonian Barton and
Becton Formations from the Hampshire Basin (Bujak et al., 1980; De
Coninck, 1995). Since the tropical bio-marker taxa used in the
international time scale are still present in the North Sea Basin
during the Eocene, biostratigraphic correlation provides consistent
anchor-points.
In the Oligocene, biostratigraphic correlations of the North Sea Basin
successions to the international time scale is hampered due to its more
restricted, semi-enclosed setting and to temperature barriers limiting
the global distribution of taxa in this icehouse time. Therefore,
calibrated ages of calcareous nannoplankton and dinocyst events are
compared to independent dating techniques such as radiometric K-Ar
dating, strontium isotope stratigraphy (SIS) and cyclostratigraphy. All
methods confirm the presence of a hiatus in between the Rupelian and
Chattian Stages in their type section and suggest a slightly younger
age for the base of Chattian than currently proposed in the
international time scale. 87Sr/86Sr ratios produce unique numeric age
solutions for nine horizons within the Rupelian and Chattian
successions that are only slightly older than the ones established by
biostratigraphic correlations. Radiometric K-Ar dates generated on
glauconites yield an apparent age of 27.0 (± 0.3) Ma for the very base
of the Chattian. This datum is within error identical to the dates
generated by Sr-isotope stratigraphy and micropaleontology.
Astronomical tuning of the early Oligocene Boom Clay succession
supports correlation of clay/silt couplets to 41 kyr obliquity cycles
and showed the presence of ~100-
and 405-kyr eccentricity cyclicity superimposed on this. Hence, it is
possible to assess the total elapsed time for deposition of the
Rupelian historical stratotype section and hence to generate a
‘floating’ time frame. Tying this relative time frame to the
international geochronologic time scale was performed by using
extrapolated ages from bio-events and strontium isotope data. This
preliminary astronomical tuning (Abels et al., in press) could be
shifted down- or upward by two 405-kyr eccentricity cycles.
Hence, multiple dating techniques infer a much younger age for the base of the
Chattian Stage than the one listed in the international time scale. The
very first late Oligocene sediments deposited during the Chattian
transgression in the North Sea Basin have an age of 27.3 to 27.0Ma.
Reconstruction of paleoenvironmental changes of Eocene-Oligocene
successions of the southern North Sea Basin was performed using benthic
foraminiferal biofacies analysis
and oxygen isotope data from benthic foraminifera, nuculid bivalves and
fish otoliths. Hence, this study provides the indispensable link
between high resolution benthic foraminiferal biofacies analyses and a
quantitative control derived from the d18O analyses.
Benthic foraminiferal biofacies are characterized by a distinct set of foraminiferal
indices (planktonic/benthic ratio, a-diversity index, absolute
abundance etc.) and by a specific assemblage of individual taxa
providing information about the depositional
environment of the fossil setting. For the Eo-Oligocene North Sea Basin assemblages, the deep water eco-group consists of e.g. Eggerella spp., Globocassidulina subglobosa,
Ehrenbergina spp., Pyrgo bulloides and Hoeglundina elegans, whereas shallow water taxa are represented by e.g. Protelphidium spp., Elphidiella spp., Asterigerinoides spp. and Quinqueloculina
spp. Relative abundance changes of these eco-groups hence reflect
changes in depositional depth. Quantification of sea level changes from
depth ranges of benthic foraminifera often leads to anomalously high
depth reconstructions for the shallow marine North Sea Basin, and
therefore, an appeal is made to oxygen isotopes. These provide a
minimum estimate for absolute (eustatic) sea level changes in the
Oligocene, by comparing the amplitude of d18O variation in between the
different
depositional sequences and assuming a constant ice volume effect.
Integration of all these different parameters leads to a robust
paleobathymetric reconstruction of the upper Eocene to Oligocene North
Sea Basin successions. Most important sea level
drops are recorded at the transition from the NS-Ru1 to the NS-Ru2 sequence,
near the base of the Ruisbroek Member, with a minimum sea level fall of ~23m.
Over the course of the Rupelian, several phases of deeper environments are
recorded near the S10 level and towards the red bed, after which the
environment gets shallower towards the double band. In the upper part
of the Rupelian, gradual shallowing leads to a more isolated marine
environment. A renewed opening of a marine pathway and the
re-installation of normal marine conditions occurs above level S190
(base NS-Ru4). Enhanced reworking of silicified upper Cretaceous
foraminifera in the upper part of the section suggests a tectonic pulse
influencing the entire southern North Sea Basin. The Rupelian-Chattian
transition in the southern North
Sea Basin is characterized by a distinct drop paleobathymetry (minimum
~14 m). Sea level gradually increases again within the Chattian Voort
Formation, with inferred
water depth of 10 to 30m.
Mean annual temperature changes are assessed by temperature sensitive benthic
foraminifera and by d18O-data from biogenic carbonate. The oxygen
isotope curve from the Middle Eocene to Upper Oligocene southern North
Sea Basin successions is in
good agreement with the global oxygen isotope curve recovered from deep-sea records
(e.g. Miller et al., 1987; Abreu and Anderson, 1998; Zachos et al.,
2001). Temperatures gradually evolve from very warm in the Middle
Eocene (~23°C) to much colder in the Oligocene (~12°C). Climate
deterioration is accelerated from the Priabonian on, and passes through
an overshoot condition after the first Oligocene
sequence (NS-Ru1) leading to a sudden, but step-wise decrease in mean
annual bottom water temperatures in the North Sea Basin. This trend is
mimicked by temperature sensitive benthic foraminifera: a first climate
deterioration is recorded near the top of the Bassevelde 1 submember, a
second within the Watervliet Member and a third very distinct
temperature drop is associated with the base of the Ruisbroek Member.
The first temperature drop is believed to correlate with a short
cooling event reported from Southern Ocean and Italian sections at
~35.5 Ma (Vonhof et al., 2000), possibly triggering a sea level
fall in between the Bassevelde 1 and Bassevelde 2 submembers
(NS-Pr1 and NS-Pr2 sequences). The increase in mean d18O-values
in between the Watervliet and Ruisbroek Member (~1.35‰ amplitude
between NS-Ru1 and NS-Ru2 sequences) is attributed to the Oi-1
glaciation, dated at 33.5 to
33.05 Ma (Zachos et al., 1996) and marking a rapid expansion of
continental ice sheets and cooling of earliest Oligocene bottom waters.
Superimposed on this sudden drop in mean annual temperature, a dramatic
shift in seasonality occurs at the same level. Mean annual range in
temperature was derived by d18O -data from incremental sampling of fish
otoliths. Seasonality evolves from >8°C during the Eocene and
earliest Oligocene to <4°C in the remainder part of the
Oligocene. This decrease in seasonality near the Eocene-Oligocene
boundary interval is in strong contrast with findings from the US Gulf
Coastal Plain (e.g. Ivany et al., 2000). A possible explanation for the
discrepancy in Oligocene climate regimes lies in the constitution of
the Gulf Stream at that time. The expansion of the polar ice sheets
across the Eocene-Oligocene boundary interval disturbs the eastward
path of the warm Proto-Gulf Stream, and resulted in much colder summer
temperatures in the North Sea Basin compared to the US Gulf Coastal
Plain.
Cool temperature conditions persisted over much of the lower Rupelian
Boom Formation, with high frequency glacial and interglacial cycles
occurring throughout. Although a relationship could not be established
between the obliquity controlled
alternation of clay/silt beds and the variations in d18O values,
prominent glacial episodes – witnessed by local d18O maxima – are
positioned at in phase 100 kyr and 400 kyr eccentricity maxima,
occurring around 31.7, 31.5 and 30.55 Ma (bed 11, 16 and S50).
Near the base of the Chattian Voort Formation, very positive d18O
values (~1.4‰) suggest a possible correlation with the Oi-2b glaciation
recorded from deep-sea settings (Wade and Pälike, 2004). This confirms
earlier suggestions by Van Simaeys et al. (2005a) that the
Rupelian-Chattian unconformity in the North Sea Basin
is genetically related to this glacial episode: this triggers a sea
level lowering and induces a hiatus of ~500 kyr in between the Rupelian
and Chattian Stages (Van Simaeys, 2004a; Van Simaeys et al., 2005a).
Within the Voort Formation, a dramatic increase of warm water taxa
(90%) is witnessed, suggesting that the earliest Chattian transgression
was associated with a
widespread major warming event. This led to the suggestion that the
early Chattian transgression in the North Sea Basin could be related to
the negative d18O
excursion recorded in the global oxygen isotope curve, and termed Late Oligocene
Warming Event (LOWE, Zachos et al., 2001). However, calculated
paleotemperatures based on benthic foraminifera and fish otoliths
indicate a cold to cold temperate temperature regime of 9 to 10°C for
the basal Chattian sediments. The marked difference and offset between
the two proxies remains intriguing and not well understood. A short
lived surface water pulse could introduce subtropical fauna, such as
the ‘Asterigerina Horizon’ in the North Sea Basin, without leaving an
imprint on the bottom water oxygen isotope signature. An
underestimation of temperatures calculated from oxygen isotopes due to
enrichment in 18O, is very unlikely, since there is no evidence for
elevated evaporation or early diagenesis inducing such anomalies.
Additional work using other independent paleotemperature proxies (e.g.
Mg/Ca; Lear et al., 2000, 2004) will be necessary to unravel late
Oligocene climate conditions. The relationship between the deep-sea
oxygen isotope excursion (LOWE) and the paleo-ecological response on
shelf communities should be evaluated by similar studies on other shelf
sections around the world. Here, it seems particularly important to
directly link paleontological temperature proxies with oxygen isotope
data and appraise the impact of deep-sea warming on shelf settings.
Integration of stratigraphic and paleoenvironmental data leads to the
establishment of a comprehensive sequence stratigraphic framework.
Seven Oligocene sequences are
newly defined or redefined for the southern North Sea Basin
successions, formalizing each sequence with adequate information about
age assessment, depositional environment and oxygen isotope
characteristics. These sequences are subsequently correlated to the
global eustatic sea level curve of Haq et al. (1987) and to the global
benthic d18O records recovered from deep-sea sections. Improved
stratigraphic calibration of the southern North Sea Basin successions
allowed establishing a causal link between the Rupelian-Chattian
unconformity and the distinct oxygen isotope excursion termed Oi2b.
This implies that the third order TA4.5/TB1.1 sequence boundary between
the Rupelian and Chattian Stages is correlating with this Oi2b global
cooling event (Van Simaeys et al., 2005a). Glacio-eustatic control for
the sequence boundary between the NS-Ru1 and NS-Ru2 sequence is
confirmed by oxygen isotopes: the increase in d18O values near the base
of the Ruibroek Member corroborates a correlation with the Oi-1
glaciation. Within the Rupelian Boom Formation, improved age
constraints suggest correlation of other sequence boundaries with
globally identified Oi-glaciations, although oxygen isotopes do not
show distinct positive increases at these levels.
The Eo-Oligocene deposits in the shallow marine North Sea Basin are a good working
ground to prove that there is a strong link between stratigraphy and
climate change. Glacio-eustatic sea level control connects both study
areas and can hence serve as a useful tool for interbasin correlations.
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Project number:
3E060245
Duration of the project:
01.10.2002 - 17.11.2006
Onderzoek met eigen middelen
Nederlands
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