Ocean
Drilling Program Leg 171B drilling
ODP Leg 171B was designed
to recover a series of 'critical boundaries' in the Earth's history during which
abrupt changes in climate and oceanography coincide with often drastic changes
in the Earth's biota. Some of these events, such as the Cretaceous-Palaeogene
(K-P) extinction and the late Eocene tektite layers, are associated with the
impacts of extraterrestrial objects, such as asteroids or meteorites, whereas
other events, including the benthic foraminifer extinction in the late
Palaeocene and mid-Maastrichtian events, are probably related to intrinsic
features of the Earth's climate system. Three of the critical intervals, early
Eocene, the Cenomanian-Turonian boundary interval and the late Albian, are
characterized by unusually warm climatic conditions when the Earth is thought to
have experienced such extreme warmth that the episodes are sometimes described
as 'super-greenhouse' periods. The major objectives of Leg 171B were to recover
records of these critical boundaries, or intervals, at shallow burial depth
where microfossil and lithological information would be well preserved, and to
drill cores along a depth transect where the vertical structure of the oceans
during the boundary events could be studied. The recovery of sediments
characterized by cyclical changes in lithology in continuous Palaeogene or
Mesozoic records would help to establish the rates and timing of major changes
in surface and deep-water hydrography and microfossil evolution.
Accordingly, five sites
were drilled down the spine of the Blake Nose, a salient on the margin of the
Blake Plateau where Palaeogene and Cretaceous sediments have never been deeply
buried by younger deposits (Fig. 1).
The Blake Nose is a gentle ramp that extends from c. 1000 to c. 2700 m water
depth and is covered by a drape of Palaeogene and Cretaceous strata that are
largely protected from erosion by a thin veneer of manganiferous sand and
nodules. We recovered a record of the Eocene and Palaeocene epochs that, except
for a few short hiatuses in mid-Eocene time, is nearly complete. Thick sequences
through the Maastrichtian, Cenomanian and Albian sequences have allowed us to
create high-resolution palaeoclimate records from these periods. The continuous
expanded records from Palaeogene and Cretaceous time show Milankovitch-related
cyclicity that provides the opportunity for astronomical calibration of at least
parts of the time scale, particularly when combined with an excellent
magnetostratigraphic record and the presence of abundant calcareous and
siliceous microfossils. Our strategy was to drill multiple holes at each site as
far down as possible to recover complete sedimentary sequences by splicing
multisensor track (MST) or colour records.
The sedimentary record at
Blake Nose consists of Eocene carbonate ooze and chalk that overlie Palaeocene
claystones as well as Maastrichtian and possibly upper Campanian chalk (Fig.
2). In turn, Campanian strata rest unconformably upon Albian to
Cenomanian claystone and clayey chalk that appear to form a conformable sequence
of clinoforms. A short condensed section of Coniacian-Turonian nannofossil
chalks, hardgrounds and debris beds is found between Campanian and Cenomanian
rocks on the deeper part of Blake Nose. Aptian claystones are interbedded with
Barremian periplatform debris, which shows that the periplatform material is
reworked from older rocks. The entire middle Cretaceous and younger sequence
rests on a Lower Cretaceous, and probably Jurassic, carbonate platform that is
more than 5 km thick in the region of Blake Nose (Shipley et al. 1978;
Dillon et al. 1985; Dillon & Popenoe 1988).
Seismic records show the
presence of buried reef build-ups at the landward end of the Blake Nose (Fig.
3). Fore-reef deposits and pelagic oozes, built seaward of the reef
front, rest on relatively flat-lying Barremian shallow-water carbonates and
serve largely to define the present bathymetric gradient along Blake Nose
(Benson et al. 1978; Dillon et al. 1985; Dillon & Popenoe
1988). Single-channel seismic reflection data (SCS) lines collected by the Glomar
Challenger over Deep Sea Drilling Project (DSDP) Site 390 and our
reprocessed version of multichannel seismic reflection (MCS) line TD-5 show that
more than 800 m of strata are present between a series of clinoforms that
overlap the reef complex and the sea bed. ODP Leg 171B demonstrated that most of
the clinoform sequence consists of Albian-Cenomanian strata and that a highly
condensed sequence of Santonian-Campanian rocks is present in places between the
lower Cenomanian and the Maastrichtian sequences (Norris et al. 1998).
The Maastrichtian section is overlapped by a set of parallel, continuous
reflectors interpreted as being of Palaeocene and Eocene age that become
discontinuous updip. Most of the Eocene section is incorporated in a major
clinoform complex that reaches its greatest thickness down dip of the Cretaceous
clinoforms.
ODP Leg 171B recovered
upper Aptian-lower Albian sediments at Site 1049 consisting of green, red, tan
and white marls overlying Barremian and Aptian pelletal grainstones and
carbonate sands. A similar succession is present at DSDP Sites 390 and 391
drilled during DSDP Leg 44 as well as in DSDP holes drilled on the Bahama
platform. The distal equivalents of these sediments are present at DSDP Site 387
on the Bermuda Rise, where they are light grey limestones interbedded with chert
and green or black claystone deposited at water depths of over 4 km (Tucholke
& Vogt 1979).
On Blake Nose, the
multicoloured claystones contain a single, prominent laminated black shale bed (sapropel)
about 46 cm thick (Fig. 4). The
sapropel is laminated on a millimetre scale and contains pyrite nodules and thin
stringers of dolomite and calcite crystals. Total organic carbon content ranges
from c. 2 wt % to over 11.5 wt % and has a hydrogen index typical of a
type II kerogen (Barker et al. this volume). A bioturbated interval of
marl about 1 cm thick occurs in the middle of the black sapropel. The lower
contact of the sapropel is gradational into underlying green nannofossil
claystone. In contrast, the upper contact is sharp below bioturbated olive green
nannofossil claystone. The planktonic foraminifer assemblage (characteristic of
the upper Hedbergella planispira Zone) together with the nannoflora
(representing biozone CC7c) suggest an early Albian age and a correlation with
OAE 1b.
Ogg et al. (1999)
estimated sedimentation rates during and after deposition of the OAE 1b sapropel.
Spectral analysis of physical property records suggests that the dominant colour
cycles (between red, green and white marls) are probably related to the
eccentricity and precession cycles and yield average sedimentation rates of c.
0.6 cm ka-1 across the OAE. Sapropel deposition persisted for at
least c. 30 ka (Ogg et al. 1999). This duration for OAE 1b is
probably an underestimate, as it is common for organic matter in sapropels to be
partly removed once oxic conditions return to the sea floor (e.g. Mercone et
al. 2000). Therefore, the OAE 1b sapropel may originally have been thicker
and represented a longer period of disaerobic seaoor conditions than is at
present the case.
By any measure, the OAE 1b
sapropel represents a remarkably long interval of disaerobic conditions.
Sapropels formed during Plio-Pleistocene time in the Mediterranean basins were
deposited on time scales of no more than a few thousand years (e.g. Mercone
et al. 2000). The best studied example of OAE 1b in Europe is the Niveau
Paquier in the Southeast France Basin. There, the upper Aptian-lower Albian
sequence is characterized by numerous black shales, all of which, with the
exception of OAE 1b, are not present at Blake Nose. On the basis of the ecology
and distribution of benthic foraminifers, Erbacher et al. (1999)
demonstrated the synchroneity of OAE 1b between Blake Nose and France.
The distribution of the
foraminifera was extensively studied across OAE 1b by Erbacher et al.
(1999) and Holbourn & Kuhnt (this volume). Sediments deposited before OAE 1b
contain a low-diversity fauna of opportunistic phytodetritus feeders:
foraminifera that feed on the enhanced carbon flux to the ocean floor. This
unique fauna replaced a highly diverse fauna and is indicative of the large
environmental changes leading to the OAE 1b. The finely laminated black shale
itself contains a very impoverished microbiota indicating disaerobic conditions
at the sea floor. Although faunal turnovers have been found to be associated
with the OAEs, Holbourn & Kuhnt (this volume) concluded from their
foraminiferal distributional study that there were no foraminiferal extinctions
associated with OAE 1b.
Erbacher et al. (in
press) showed new stable isotope data on foraminifera from early Albian OAE 1b.
Those workers demonstrated that the formation of OAE 1b was associated with an
increase in surface-water runoff, and a rise in surface temperatures, that led
to decreased bottom-water formation and elevated carbon burial in the restricted
basins of the North Atlantic. The stable isotope record has similar features as
the Mediterranean sapropel record from the Pliocene-Quaternary period inasmuch
as there is a large negative shift in 
18O
of planktic foraminifera in both instances that probably reflects the freshening
and perhaps temperature rise associated with the onset of sapropel formation.
However, the geographical extent of the OAE 1b is much larger and its duration
is at least four times longer than any of the Quaternary sapropels. Hence,
current results suggest that OAE 1b was associated with a pronounced increase in
surface-water stratification that effectively restricted overturning over a
large portion of Tethys and its extension into the western North Atlantic.
A thick section of
Albian-Cenomanian continental slope and rise sediments were recovered in Holes
1050C and 1052E on Blake Nose. In Hole 1052E, 215 m of black and green laminated
claystone, minor chalk, limestone and sandstone were recovered, representing the
uppermost Albian and lowermost Cenomanian interval. Sandstones recovered at the
base of the hole give way to dark claystones with laminated intervals and thin
bioturbated limestones higher in the sequence. A partly correlative sequence was
recovered in Hole 1050C where the section starts in latest Albian time and
continues through most of Cenomanian time. The Turonian, Santonian and Coniacian
periods are represented in a highly condensed sequence of multi-coloured chalk
just above the last black shales and chalks of the Cenomanian sequence.
The upper Albian-lower
Cenomanian strata in Hole 1052E preserve a biostratigraphically complete
sequence from calcareous nannofossil Zone CC8b to CC9c ( planktonic foraminifer
biozones Rotalipora ticinensis to R. greenhornensis). The time
scales of Gradstein et al. (1995) and Bralower et al. (1997a) and
the biostratigraphy from Site 1052 suggest the sequence records c. 8 Ma
of deposition between c. 94 and c. 102 Ma. Sedimentation rates
drop off dramatically in the upper c. 25 m of the Cenomanian sequence
about 98 Ma. The remainder of the sequence was deposited within a 2 Ma interval
at sedimentation rates of about 9-10 cm ka-1. The termination of
these high sedimentation rates coincides with a lithological change from black
shale to chalk deposition, probably reflecting the sea-level rise in early
Cenomanian time.
The section across the
Albian-Cenomanian boundary in Hole 1052E is correlative with OAE 1d. Black and
green shales become increasingly well laminated and darker in colour approaching
the Albian-Cenomanian boundary and are accompanied by interbeds of white to grey
limestone. High-resolution resistivity (Formation Microscanner; FMS) logs from
Hole 1052E demonstrate that the OAE is not a single event, but represents a
gradual intensification of the limestone-shale cycle that terminated abruptly at
the end of the OAE (Kroon et al. 1999; Fig.
5).
Cyclostratigraphy suggests
there is a lowfrequency cycle (seen best in the unfiltered FMS log) with a
wavelength of c. 9-10 m that is expressed in cycles of increasing and
then decreasing FMS amplitude. In turn, the 10 m cycle consists of bundles of
1.8-2 m cycles. Filtering the FMS record (that is sampled at better than 1 cm
resolution) with a 70 cm-20 m window reveals these dominant cycles clearly.
There is a still higher frequency cycle about 10-15 cm wavelength that is
particularly well expressed in the claystone interbeds and are expressed by high
FMS resistivity. These high-resistivity claystones contrast with the low
resistivity of more carbonate-rich beds, including the limestone beds that are
prevalent in the interval around the Albian-Cenomanian boundary.
The ratios of the 10 and
~2 m cycles are about right to represent the 100 ka and c. 21ka bands of
orbital precession (Fig. 5). The
higher-frequency cycles do not correspond neatly to orbital bands. We suggest
that either our initial guess is wrong that the longer-wavelength cycles
represent eccentricity or differential compaction around the limestone stringers
has altered the relative spacing of cycles sufficiently to obscure the duration
of the high-frequency cycles. However, if we assume that the low-frequency cycle
is in the eccentricity band, we can estimate an average sedimentation rate of c.
9.0-10.0 cm ka1. The whole interval represented by the increasing
cycle amplitude to the termination of the OAE (between c. 575 and 506
mbsf (metres below sea floor)) represents a little over a 750 ka interval of
which the peak of OAE 1d lasts for c. 400 ka of earliest Cenomanian time.
Norris & Wilson (1998)
have shown that the planktonic foraminifera found in the shale beds are
extremely well preserved and record an original stable isotopic signature of the
oceans across OAE 1d. They found that the subtropical North Atlantic had sea
surface temperatures on average warmer than today at between 30 and 31oC.
Temperatures peaked at about the level (c. 510 mbsf) where the maximum
amplitude of the FMS cycle is also recorded. Apparently the increase in
intensity of the OAE was directly mirrored by a rise in surface temperatures.
Temperature and the vertical thermal gradient both collapsed near the
termination of the OAE, suggesting a fundamental change in ocean circulation and
stratification near or at the end of OAE 1d.
The Cenomanian-Turonian
boundary interval was investigated at Site 1050 at Blake Nose by Huber et al.
(1999). This important interval is characterized by one of the major Cretaceous
Anoxic Events (OAE 2). Although the sequence at Site 1050 is not entirely
complete, Huber et al. (1999) were able to measure stable isotopes of
unaltered calcareous planktonic and benthic foraminifera across the
Cenomanian-Turonian transition. One of the astonishing results is the massive
warming of middle bathyal temperatures based on benthic oxygen isotope results.
Bathyal temperatures were already rather high (15oC) before the
boundary but rose to about 19oC within OAE 2. This deep-water warming
does not seem to be mirrored by sea surface water temperatures and therefore
(Huber et al. 1999) concluded that most heat during OAE 2 time, a
super-greenhouse event, was transported via the deep ocean. The warming may have
been responsible for the extinction of deeper-dwelling planktonic foraminiferal
genera such as Rotalipora.
The calcareous nannofossil
stratigraphy shows that the Maastrichtian sediments, although slumped in parts,
are biostratigraphically complete (Self-Trail this volume). The mid-Maastrichtian
interval is characterized by a series of important biological and
palaeoceanographic events. Extinction of deep-sea inoceramid bivalves and
tropical rudist bivalves occurred at the same time with geochemical shifts in
deep-sea biogenic carbonates and a pronounced cooling of high-latitude surface
waters. It is not clear how all these events are related. Blake Nose middle
Maastrichtian sediments, although complicated in places by slumping and coring
gaps, yield conclusive evidence in the form of stable isotopes from foraminifera
and extinctions concerning the subtropical palaeoceanographic evolution of the
area. In contrast to cooling at high-latitude sites, Blake Nose surface waters
show a temperature rise of about 4oC (MacLeod & Huber this
volume). Blake Nose results highlight regional mid-Maastrichtian differences.
The benthic foraminiferal oxygen isotopes do not show a marked shift in mid-Maastrichtian
time in contrast to southern Ocean and Pacific stable isotope records.
Therefore, MacLeod & Huber (this volume) excluded build up of ice during the
course of Maastrichtian time as the force behind the chain of mid-Maastrichtian
events, although the details of mid-Maastrichtian palaeoceanography and cause of
extinctions remain enigmatic.
The ODP Leg 171B recovered
a conspicuous Cretaceous-Palaeogene (K-P) boundary interval at Blake Nose (Fig.
4). At the deepest Site 1049 three holes were drilled through the K- P
interval. The boundary layer, mostly consisting of green spherules, was
interpreted to be of impact origin and ranges in thickness from 6-17 cm in the
three holes at Site 1049. The largely variable thickness suggests reworking of
the ejecta material down slope after deposition. Martínez-Ruiz et al. (this
volume a & b) have shown that the green spherules represent the
diagenetically altered impact ejecta from Chicxulub. Martínez-Ruiz et al. (this
volume b) described the now predictive sequence of meteorite debris and
associated chemistry across the K-P boundary at Blake Nose: the impact-generated
coarse debris or tektite bed is followed by finegrained pelagic ooze of the
earliest Danian period rich in iridium.
One of the remarkable
features of the pre-impact sediments at Blake Nose is deformation and
large-scale slope failures probably related to the seismic energy input from the
Chicxulub impact, some of it clearly induced before the emplacement of the
ejecta from the impact (Norris et al. 1998; Smit 1999; Klaus et al. 2000).
Mass wasting as a response to the impact occurred at a large scale. Correlation
between Blake Nose cores and seismic reflection data indicates that the K-P
boundary immediately overlies seismic facies characteristic of mass wasting
(Klaus et al. 2000). Sediments 1600 km away from the Chicxulub impact on
the Bermuda Rise were 'shaken and stirred'. Norris et al. (this volume) found
that the seismic reflector associated with mass wasting at the K-P boundary is
found over nearly the entire western North Atlantic basin, suggesting that much
of the eastern seaboard of North America had catastrophically failed during the
K-P impact event. Norris et al. (unpubl. data) made a survey of rise and
abyssal sediment cores off North America, Bermuda and Spain, and concluded that
indeed mass wasting may well have disrupted pelagic sedimentation at the K-P
boundary in all those places.
The biostratigraphy of the
K-P interval at Blake Nose is typical of an Atlantic seaboard deep-sea K-P
section (Norris et al. 1999). Ooze immediately below the spherule bed
contains characteristic late Maastrichtian planktonic foraminifera and
nannofossils. The ooze above the spherule bed contains abundant extremely small
Palaeocene planktic foraminifers in addition to large Cretaceous foraminifera.
Norris et al. (1999) argued that the large Cretaceous planktonic
foraminifera found in Darian sediments have been reworked. Small specimens of
the same Cretaceous species are hardly present in the post-impact ooze, which
implies that some form of sorting has changed the usual size distribution of
planktonic foraminifera, which tends to be dominated by small individuals in
pelagic sediments. The important conclusion is that the impact ejecta exactly
coincided with the biostratigraphic K-P boundary and the planktonic
foraminiferal extinction was caused by the Chicxulub impact.
Drilling at Blake Nose
recovered a continuous sequence of the Palaeocene-Eocene transition at a
relatively low-latitude site. The upper Palaeocene section at Site 1051 consists
of greenish grey siliceous nannofossil chalk that exhibits a distinctive colour
cycle between 23-29 cm wavelength in Palaeocene time and c. 1 m
wavelength in latest Palaeocene and early Eocene time. A distinctive bed of
angular chalk clasts, now deformed by compaction, occurs at about the level at
which the increase in colour cycle wavelength is seen. Norris & Röhl (1999)
found that the LPTM as defined by a c. 2-3
13C
excursion and the appearance of a distinctive 'excursion fauna' of planktic
foraminifera occurs just above the chalk breccia. Furthermore, Katz et al. (1999)
found the extinction of an assortment of cosmopolitan benthic foraminifera in
site 1051 that are also known to have become extinct at the LPTM in other
regions around the world. Hence, we are confident that we recovered a section
typical of the LPTM on Blake Nose.
Bains et al. (1999),
Katz et al. (1999) and Norris & Röhl (1999) have interpreted the
chalk breccia horizon, and step-like features in the 13C
anomaly as strong evidence that the LPTM was associated with rapid release of
buried gas hydrates. Oxidation of released methane to CO2 would have
caused a rapid increase of this greenhouse gas in the atmosphere and thus
resulted in warming of the planet. Bains et al. (1999) showed that the
onset of the carbon isotope anomaly occurred in a series of three pronounced
drops in 13C
separated by plateaux (Fig. 6).
They interpreted the steps and intervening plateaux in 13C
as intervals of rapid methane outgassing separated by intervals where the rate
of carbon burial roughly balanced its rate of release into the ocean and
atmosphere. Katz et al. (1999) showed that the chalk clast breccia can be
reasonably interpreted as a debris flow produced by methane hydrate
destabilization and venting during the initial phases of the LPTM.
The continuous sequence of
Blake Nose across the Late Palaeocene Thermal Maximum (LPTM) displays very
pronounced cyclicity of the sediments shown in spectral reflectance records at
Site 1051. The carbon isotope anomaly within C24r coincides with a major shift
in lithology and cyclicity of various physical property records including
sediment colour, carbonate content and magnetic susceptibility. Norris &
Röhl (1999) used the cyclicity to provide the first astronomically calibrated
date for the LPTM (c. 54.98 Ma) and a chronology for the event itself
using the Milankovitch induced cyclostratigraphy at Site 1051. Kroon et al. (1999)
showed that the LPTM may be part of a long-term climatic cycle with a wavelength
of 2 Ma by using the downhole gamma-ray log (Fig.
7). The LPTM or carbon anomaly coincides exactly with one of the
gamma-ray maxima, possibly showing increased influx of terrigenous clay or less
dilution by relatively reduced carbonate content.
Dickens (this volume)
reported the results of a numerical modelling study designed to evaluate the
potential range for the mass of carbon released. Using the 13C
record from Site 1051 as the target site, he used a simple box model to assess
the implications of variations in the mass and/or isotopic composition of the
primary carbon fluxes and reservoirs. Dickens (2000b) concluded that a massive
release of 1800 Gt of methane hydrate explains best the carbon isotope
excursion.
The Eocene sequence
consists largely of green siliceous nannofossil ooze and chalk. The upper part
of the sequence is typically light yellow, and the colour change to green
sediments below is sharp. This colour change is diachronous across Blake Nose
and probably relates to a diagenetic front produced by flushing the sediment
with sea water. Planktonic foraminifers, radiolarians and calcareous
nannofossils are well preserved throughout most of the middle Eocene sequence,
but calcareous fossils are more overgrown in the lower middle Eocene and lower
Eocene sequence. A distinct feature of the Eocene sequence is a high number of
vitric ash layers that were found at each site. These ash layers can be
correlated across and serve as anchor points within the highly cyclical (at the
Milankovitch scale) sequences. One of the conspicuous dark upper Eocene layers
at Site 1053 contains nickel-rich spinels, which indicates that this layer
contains potentially extraterrestrial material as a consequence of the
Chesapeake Bay impact (Smit, pers. comm.). The Palaeocene and lower Eocene
sediments are relatively clay rich compared with the middle and upper Eocene
deposits. The upper Palaeocene sequence contains chert or hard chalk, and
preservation of most fossil groups is moderate to poor. The lower Palaeocene
sediment is typically an olive green, clay-rich nannofossil chalk or ooze.
Calcareous microfossils are typically very well preserved, whereas siliceous
components are absent.
A number of excellent
biostratigraphic studies have developed from the Blake Nose Palaeogene records.
The preservation of the mid- and late Eocene calcareous fossils is very good and
the preservation of the siliceous fossils is adequate to construct a detailed
biostratigraphy, one of the first for late Palaeocene and early Eocene time.
Also, the organic-walled
fossils including the dinoflagellate cysts appear to be very useful. The good
preservation of both carbonate and siliceous microfossils led to the first
well-integrated biostratigraphy of mid-latitude faunas and floras (Sanfilippo
& Blome this volume). The sedimentary sequence is unique in containing
well-preserved radiolarian faunas of late early Palaeocene to late mid-Eocene
age. Sanfilippo & Blome (this volume) have documented 200 radiolarian
biohorizons. Middle to Upper Eocene sediments contain dinocyst assemblages
characteristic of warm to temperate surface waters (van Mourik et al. this
volume). The absence of known cold-water species shows there was no influence of
a northern water mass in the area of Blake Nose and thus subtropical conditions
prevailed throughout mid-late Eocene time.
One of the main objectives
for drilling Blake Nose was to recover complete sequences of Palaeogene
sediments by drilling multiple holes. The recovery of continuous sequences
characterized by cyclical changes in lithology would help to establish the rates
and timing of major changes in surface and deep-water hydrography. Röhl et
al. (this volume) have presented a study on the use of Milankovitch forcing
of sediment input, expressed in the periodicity of iron (Fe) concentrations, to
calculate elapsed duration of the Danian stage. A new astronomical Danian time
scale emerged from counting obliquity cycles at Sites 1050C and 1001A (Caribbean
Sea), which surprisingly appeared to be the dominant frequency in the record.
Röhl et al. elegantly made use of the Fe records observed in the cores and
various downhole logs to fill in for drilling gaps. The results from both
drilling sites were neatly similar. Analysis of the cyclostratigraphy both from
the cores and downhole logs appears to be successful in the older parts of the
stratigraphy and useful in calibrating magnetic polarity zones.
Wade et al. (this
volume) have presented the first detailed benthic and planktonic foraminiferal
oxygen isotope curves in combination with spectral reflectance records from late
mid-Eocene time. Those workers found clear evidence of Milankovitch cyclicities
at Site 1051 (Fig. 8; Wade et al. this volume). At Site 1051 in the top
30 m, the colour records document a dominant cyclicity with wavelengths of
1.0-1.4 cycles m-1 forced by precession (Fig.
8), whereas the stable isotope records demonstrate all frequencies of
the Milankovitch spectrum. Changes in the 18O
records of the surface-dwelling planktonic foraminifera are astonishingly large.
Wade et al. (this volume) concluded that upwelling variations may be
responsible for these large variations resulting in large sea surface
temperature variations. Sloan & Huber (this volume) in a modelling study
also found that changes in wind-driven upwelling and in continental runoff on a
precessional time scale should be observed in regions of the central North
Atlantic.
Another subject relates to
the distribution of clay minerals. One of the most exciting aspects is that the
origin of palygorskite clay minerals found in lower Eocene pelagic sediments in
the western Central Atlantic may be authigenic (Pletsch this volume). Of
particular interest and greater potential significance is the idea that
authigenic palygorskite formation required the presence of relatively saline
bottom waters, such as the elusive 'Warm Saline Bottom Water' (WSBW) of Brass et
al. (1982). As such the distribution of palygorskite could become a
palaeo-watermass tracer of WSBW.
