Netherlands Journal of Geosciences 81 (3): 289-302 (2002)
Diagenesis, regular growth and records of seasonality in inoceramid
bivalve shells from mid-Maastrichtian hemipelagic beds of the Bay of Biscay
J.J. Gómez-Alday1 & J. Elorza1
1 Departamento de Mineralogía-Petrología, Universidad del
País Vasco, Apartado 644, E-48080 BILBAO, Spain; e-mail: nppelzaj@lg.ehu.esManuscript
received: 3 July 2000; accepted in revised form: 15 September 2001
Abstract
Inoceramid bivalve shells from outcrops of mid-Maastrichtian deep-water
carbonate, hemipelagic beds in the Bay of Biscay exhibit post-depositional
diagenetic alteration. New data from isotopic analysis (carbon and oxygen),
together with observations of the inoceramid shells and carbonate host-rock
using cathodoluminescence (CL) and scanning electron microscopy (SEM),
confirm a lateral, westerly increase in the degree of diagenesis, without
any substantial textural changes in the alternating dark and clear growth
lines of the shell microstructure. Under CL, a bright yellowish to red
colour is observed in the most diagenetically altered inoceramid samples.
Non-luminescent areas are restricted to the central parts of the less
altered shells. A detailed geochemical analysis by electron microprobe,
along intrashell profiles of the non-luminescent and luminescent zones
has revealed that Mg/Ca, Sr/Ca, Na/Ca, Fe/Ca and Mn/Ca ratios show oscillatory
curves but behave differently. Fe/Ca, Mn/Ca and Na/Ca ratios are well
correlated but usually show an opposite relationship when compared with
the Mg/Ca and Sr/Ca ratios of both luminescent and non-luminescent shell
areas. Our findings have palaeoenvironmental implications in that the
geochemistry of the regular, alternating dark and clear growth lines seems
to be related to the input of seasonally controlled phytodetritus to the
basin floor.Keywords: inoceramids, deep-water setting, seasonality, stable
isotopes, minor and trace elements, diagenesis, mid-Maastrichtian, Basque-Cantabrian
basinIntroduction
Inoceramid bivalves are important Cretaceous biostratigraphical indices
because of their cosmopolitan distribution (Dhondt, 1992). Deep Sea Drilling
Project (DSDP) studies have revealed that inoceramids occurred in all
of the world oceans, at different depths (from shelf to abyssal palaeodepths)
and from tropical to austral palaeolatitudes during the Late Cretaceous
(Saito & Van Donk, 1974; Saltzman & Barron, 1982; Barron et al.,
1984; MacLeod et al., 1996). Inoceramids were epifaunal, filter-feeding
bivalves which are found associated with different carbonate or terrigenous
sediment substrates.
A variety of comparative geochemical studies (analyses of stable isotopes,
and minor and trace elements) of extant as well as different macro- and
microfossil calcareous skeletons have been published since the 1980s.
Morrison & Brand (1984, 1988), Veizer et al. (1986), Krantz et al.
(1987), Barrera & Tevesz (1990) and Barrera et al. (1990), among others,
characterised palaeoenvironmental features at different latitudes and
sedimentary settings. Studies of minor and trace element contents in the
calcareous skeletons of both fossil (Ragland et al., 1979; Brand &
Veizer, 1980; Brand & Morrison, 1987; Barbin et al., 1995; Mii &
Grossman, 1994; Grossman et al., 1996; Dauphin et al., 1996; Purton et
al., 1999) and living specimens have been published (Morrison & Brand,
1986; Barbin et al., 1991a, b; Mann, 1992; Klein et al., 1996a, b, 1997;
Jones & Quitmyer, 1996). It is generally assumed that the final composition
of fossil shells is determined by:
A - the physical/chemical environment in which the organism lived; B -
biological controls during skeletal growth; and
C - subsequent diagenetic alteration undergone by the shell (Dodd &
Stanton, 1981).
We have previously characterised the diagenetic behaviour of inoceramid
shells (paths of diagenetic fluid advance, oxygen and carbon isotope variations,
geochemical relationship with the carbonate host-rock, etc.) during progressive
burial of Upper Cretaceous sediments in the Basque-Cantabrian Basin (Elorza
& García-Garmilla, 1996, 1998; Gómez-Alday & Elorza,
1998), in the chalk facies from the Liège-Limburg basin (Belgium,
the Netherlands) (Elorza et al., 1997) and in France (Gómez-Alday
et al., 1998). Our first objective was to determine the degree of diagenetic
alteration of the new inoceramid samples collected from different sections
of the Bay of Biscay from a geochemical and petrographic point of view,
and to establish if there are petrographic and geochemical differences
between the more altered and the less altered inoceramid shells. In order
to evaluate the degree of diagenesis, we examined the inoceramid shells
together with the host-rock, under microscopy, SEM and CL. In addition,
oxygen and carbon isotopes were determined and the Mg, Ca, Mn, Fe, Sr
and Na contents of selected inoceramid shells were also examined. Our
second objective was to determine if the less altered inoceramid samples
preserve part of the primary palaeobiotic signal and geochemically determine
their regular seasonal growth pattern in deep-water environments. In the
present paper, we put forward evidence which indicates that certain geochemical
values may be close to the original composition (minor and trace elements)
of deep-water ocean inoceramid shells.Geological setting
The Sopelana I, Zumaya, San Sebastián, Hendaya, Loya and Bidart
sections are located in the so-called Basque Arc domain (Rat, 1959; Feuillée
& Rat, 1971) (see Fig. 1). They comprise well-exposed, Upper Cretaceous
sediments, which crop out along the coast of the Bay of Biscay (Spain
and France). The samples selected for this study belong to mid-Maastrichtian
carbonate-rich facies, which consists of marl and marly-limestone alternations,
each about 20-30 cm thick. The occurrence of terrigenous sediments such
as distal turbidite beds are important in the Zumaya, San Sebastián,
Loya and Hendaya sections (Fig. 2). On the basis of its trace fossil association,
benthic forams and ostracods (see Pujalte et al., 1998), the basin has
been considered to be mesobathyal (1,500-2,000 m).
Five lithostratigraphic members were recognised within the mid-Maastrichtian
of the Zumaya-Algorri Formation (MacLeod & Ward, 1990; Ward &
Kennedy, 1993) although with different thickness in the Sopelana I, Zumaya
and Bidart sections (this formation was defined by Mathey, 1982, p. 135;
and later erroneously referred to by other authors as the Zumaya-Algorta
Formation). Inoceramids are most abundant in the so-called Member I, belonging
to the Gansserina gansseri Zone, and do not reach into the Abathomphalus
mayaroensis Zone (planktonic forams). They eventually disappear in the
upper part of Member II, coinciding with nannofossil zones 24-25A, and
equating with the Anapachydiscus fresvillensis ammonite zone, in magnetochron
31N (data summarised by MacLeod & Orr, 1993; Ward & Kennedy, 1993)
(see Fig. 2).
These lithologically homogeneous sediments were deposited in a hemipelagic
environment, which persisted during the Maastrichtian (Mathey, 1982, 1987).
Six different species of inoceramids have been identified (MacLeod &
Ward, 1990; Ward et al., 1991; MacLeod, 1994). Most inoceramid shells
appear complete both in marly and marly-limestone sediments. Sometimes,
they are colonised by small oyster shells (Pycnodonte sp.). Small, oriented,
comma-shaped borings can also be observed which correspond to cirripede
endobionts such as Rogerella, and are mainly distributed following the
dark growth lines.Methods and materials
Fragments of inoceramid prismatic calcite shell material were selected
and examined under SEM using a Jeol JSM-T6400 at the Universidad del País
Vasco. All CL work employed a Technosyn Cold Cathode Luminescence system,
model 8200 Mk II, mounted on an Olympus trinocular research microscope
with a maximum magnification capability of x400, utilising universal stage
objectives. Standard operating conditions included an accelerating potential
of 12 kV and a 0.5-0.6 mA beam current with a beam diameter of approximately
5 mm.
The selected samples from the six stratigraphic sections were analysed
for oxygen and carbon isotopes. The stable isotope values of 18O/16O and
13C/12C from inoceramid shells and host-rock were determined by using
a VG SIRA-9 mass spectrometer at the Universidad de Barcelona and Universidad
de Salamanca (Spain). Extraction of CO2 from carbonates was carried out
according to the McCrea method (1950). The results are expressed in notation
in ‰, relative to the Pee Dee Belemnite (PDB) standard. Reproducibility
for both 18O and 13C is better than 0.1‰.
In addition, the inoceramid shell samples MA-33 (Sopelana I section) and
BID-1 (Bidart section), which have the lightest and heaviest oxygen isotope
values, respectively, and distinct CL were polished to determine the Mg,
Ca, Mn, Fe, Sr and Na contents by electron microprobe. A Cameca SX100
electron microprobe at the Département des Sciences de la Terre
(Université Blaise Pascal, Clermont-Ferrand, France) was used for
this purpose. Working conditions were 10 s counting time, c. 10 nA beam
current and 15 kV accelerating voltage, using a beam area of 20 µm2.
Calibration was against Bureau de Recherches géologiques et minières
(BRGM) standard minerals, and the ZAF correction program was used (Henoc
& Tong, 1978). The analyses in shells (a total of 277 spots) were
performed taking into account the thickness of the growth lines; where
it was not possible to recognise them, sampling was carried out at 20
µm intervals, which is the usual distance between two consecutive
growth lines. During analyses, we tried to avoid the longitudinal inter-prism
surfaces due to their potential ways of diagenetic calcite precipitation.
In order to facilitate comparison with other studies, the chemical composition
of inoceramid shells was also recalculated as ppm of Mg, Ca, Mn, Fe, Sr
and Na.
Results
Petrography of inoceramid shells
More than 100 thin sections were examined by standard microscopy methods.
These sections were stained with Alizarin Red S and potassium ferricyanide
(Dickson, 1965). A well-developed, prismatic calcite microstructure characterises
the inoceramid shells, which range from 1 to 6 mm in thickness (Figs 3A-H,
4A, 6A, B). In longitudinal shell sections, calcite prisms decrease in
width from the inner shell layer (ISL) to the outer shell layer (OSL).
Each inoceramid prism corresponds to a single or to various crystals and
is about 0.1 mm (ISL) to 0.01 mm (OSL) wide and between 1 and 3 mm long.
Alternating dark (laminae obscurae) and clear (laminae pellucidae) growth
lines, commonly spaced at intervals of approximately 20 µm, are
visible perpendicular to the long axes of the prisms and do not present
any appreciable internal breaks (Fig. 4A-C). Lines gradually deflect towards
the OSL. The calcite prisms are composed of low-magnesium calcite (LMC
with < 4 mol% MgCO3) and we did not observe the inner aragonitic nacreous
shell layer described by other authors from other localities (Wright,
1987; Whittaker et al., 1987; Pirrie & Marshall, 1990).
For a detailed study under SEM and CL, we chose samples from the Sopelana
I and Bidart sections since these are composed entirely of marl/marly-limestone
alternations and turbidite beds are absent. Nevertheless, the Bidart alternations
are less indurated than those of Sopelana I, which implies that compaction
and secondary calcite precipitation have been more pervasive at the Sopelana
I section. In fact, the loss of intra- and interprism porosity due to
calcite cement precipitation is visible under SEM in inoceramid shells
from the Sopelana I section (Fig. 3A, B). The calcitic prismatic layers
of the shells from the Bidart section seem to be well preserved and without
evidence of neomorphism. The ISL and OSL, perpendicular to the long axis
of the prisms, seem to consist of regular, simple prismatic calcite, with
a polygonal, honeycomb morphology and well-defined boundary surfaces (Fig.
3C, D). Very few calcite-filling cements can be observed between the prism
boundaries. The microstructure of the individual prisms exhibits a uniformly
distributed porosity.
Under CL, most of the inoceramid samples from the Sopelana I section show,
in general, well-developed bright yellowish to red luminescence in the
whole shell, and non-luminescent surfaces are absent (Fig. 6A). However,
inoceramid shells from Bidart usually show a well-developed bright yellowish
to red luminescence at shell edges and non-luminescent surfaces are visible
in the middle shell layer (MSL) (Fig. 6B). Single inoceramid shell prisms
floating in the sediment show a complete bright reddish to yellowish luminescence
as well. Hendaya and Loya samples present high quantities of fracture-filling
calcite cements with different luminescent behaviour (Fig. 3E-H).Oxygen
and carbon isotope values
The oxygen and carbon isotopes of a total of 244 samples (host-rock, n
= 130; inoceramid shell prisms, n = 114) were determined. Isotopic results
are given in Table 1. In shell prisms, the mean inoceramid oxygen and
carbon isotope values were usually heavier than in host-rock (Fig. 5).Inoceramid
shell chemistry determined by electron microprobe
The Mg/Ca, Mn/Ca, Fe/Ca, Sr/Ca and Na/Ca ratios (mmol/mol) of the inoceramid
shell samples MA-33 and BID-1 were determined. These samples were chosen
due to their extreme oxygen isotopic compositions and petrographical features
(Fig. 6A, B). Over 149 spots were analysed in sample MA-33 and 128 spots
in the inoceramid shell BID-1. We distinguished between the non-luminescent
and luminescent shell areas in order to set up chemical differences in
the distribution both areas (Table 2; Fig. 6C-E).Discussion
Inoceramid shell preservation and diagenetic behaviour
For our proposition, the inoceramid shells from the Bay of Biscay sections
require that some of the form of their original morphological structure
be found intact in marl/marly limestone couplets, and also preserve their
pristine geochemical composition, as Brand & Morrison (1987) mentioned
in their work on molluscs. However, oxygen and carbon isotope values from
inoceramid shells and carbonate host-rock samples indicate clear diagenetic
differences among the six sampled localities (Table 1). Mean oxygen isotope
values, both in inoceramids and host-rock samples, tend to be lighter
towards the western sections in response to increasing burial (Fig. 5;
see also Fig. 1). The heaviest values correspond to the Bidart section
whereas the lightest ones are found at Zumaya and Sopelana I. The Hendaya
and Loya isotope values are similar to those found at Zumaya and Sopelana
and disrupt this trend. As we have noted earlier, samples from the Hendaya
and Loya sections present high quantities of fracture-filling calcite
cements. Secondary calcite might be responsible for lowering the oxygen
isotope signal.
The replacement of primary skeletal calcite and proteic matrix by secondary
Fe-Mn rich calcite into intra and/or inter-prisms may be responsible for
decreasing the oxygen isotope values of the inoceramid samples. In the
same way, primary calcite dissolves in the carbonate host-rock, and is
replaced by secondary calcite, which forms in equilibrium with the temperature
and oxygen isotopic composition of carbonate sediment pore fluids. With
increasing burial, the geothermal gradient elevates sediment temperatures
and the oxygen isotope values of secondary calcite should be more negative
than those of the primary calcite. In contrast, carbon isotope values
are not affected during diagenesis and they exhibit low variation between
localities.
As Mii & Grossman (1994) and Grossman et al. (1996) noted, the distribution
of minor and trace element contents in non-luminescent and luminescent
areas of Pennsylvanian (Carboniferous) brachiopod shells was different.
Consequently, we selected the two shells which had the lightest (MA-33,
18O=-5.18 ‰) and the heaviest (BID-1, 18O=-0.38‰) oxygen isotope
values. During burial diagenesis, the composition of inoceramid shells
has to change in order to reach the new chemical 'equilibrium conditions'.
In this sense they obtain a trace element composition shifted towards
equilibrium with the interstitial fluids, probably seawater in origin.
The new inoceramid shell chemical composition was reached without any
substantial change in the calcite prism microstructure, which suggests
that element substitution took place atom by atom (solid-state diffusion).
This post-depositional element exchange produced chemical zoning in the
shells (Table 2). Thus, luminescent and more altered shell areas became
selectively enriched in Mn/Ca, Fe/Ca and Na/Ca and consequently lower
Mg/Ca and Sr/Ca ratios (Fig. 6C, D). On the other hand, low Mn/Ca, Fe/Ca
and Na/Ca and well-correlated Mg/Ca and Sr/Ca ratios are found in non-luminescent
(less altered) inoceramid shell areas (Fig. 6D). In general, diagenetic
re-equilibria in a closed system (low water/rock ratio) may explain why
the original oscillatory elemental distribution is partially preserved
in the luminescent shells areas. Conversely, high water/rock ratios could
lead to a more pervasive diagenesis with numerous dissolution-reprecipitation
events (Brand & Veizer, 1980). Carbon isotope values suggest that
re-equilibration of calcite with the surrounding diagenetic fluids took
place in a closed system (Emrich et al., 1970; Scholle & Arthur, 1980;
Maliva et al., 1991).
In our case, as we have pointed out before, we observed a trend in element
incorporation in both luminescent or non-luminescent areas, although a
better compositional regularity characterises the BID-1 non-luminescent
shell (Fig. 6D). We have noted that, although diagenetic processes tend
to obliterate the primary biogenic calcite signal, the original composition
of the alternating dark (laminae obscurae) and clear (laminae pellucidae)
growth lines might control the amount of element exchange. The possibly
primary Mg, Sr, Na, Fe and Mn values (as ppm) in non-luminescent sections
of the inoceramid shell BID-1 could be inferred in the same way. These
values nearly fall in the range of Mg, Sr, Na, Fe and Mn Cretaceous molluscan
and bivalve LMC contents compiled by Ragland et al. (1979) and Brand &
Morrison (1987) (Table 3).Nutritionally controlled growth pattern
In bivalves and belemnite rostra, alternating clear and dark growth lines
are interpreted as inorganic and organic-rich layers, respectively (Kennish,
1980; Sælen, 1989). It is well known that bivalve molluscs secrete
shell material in periodic growth increments, which can provide a record
of environmental conditions (productivity, temperature and salinity) during
growth. Therefore, trace element distributions in shallow-water marine
bivalves are often analysed, since the shallow-marine environment is subject
to a variety of changes in temperature and chemical conditions.
A detailed study of the clear and dark growth line arrangement, up to
500 in inoceramid sample MA-33, shows that they were commonly more densely
packed in mature shell portions (ISL), without any observable internal
breaks (Fig. 4), indicating a gradual decrease in growth rate throughout
ontogeny. This closely spaced feature is a common characteristic in deep-sea
bivalves (Gilkinson et al., 1986). In bivalves, dark organic-rich lines,
now luminescent, are often correlated with seasonal events. In general,
molluscs close their valves under adverse conditions, and have less or
no contact with the surrounding water and therefore shell accretion decreases
(Lutz & Rhoads, 1980; Barbin et al., 1991a, b; Barbin et al., 1995).
The alternating dark and clear, regularly spaced growth lines suggest
that shell accretion took place under stable and often changing environmental
conditions.
As stenotopic organisms, deep-water ocean inoceramids probably did not
experience strong environmental fluctuations (temperature and salinity)
to produce similar skeletal Mg/Ca and Sr/Ca trends as found in shallow-water
marine molluscs. Therefore, shallow-water, sessile, eurytopic molluscs
might be sufficiently flexible to change their physiological system in
response to changes in environmental conditions. Klein et al. (1996a,
b) pointed out that the Mg/Ca ratios in shallow-water molluscan shells
do not show a strong dependence on salinity but vary linearly with temperature
alternations. The Sr/Ca ratio in skeletal calcite seems to be more closely
related to mantle metabolic activity than to variations in sea-water salinity.
Klein et al. (1997) reported that the skeletal Sr/Ca ratio is positively
correlated with seawater 18O values in mature parts of the shell, where
the intracellular transport mechanism dominates, so in this shell portion,
Mg/Ca (due to its positive relationship with temperature) and Sr/Ca should
present similar trends.
On the other hand, Purton et al. (1999) found that temperature has only
a secondary influence on Sr/Ca ratios on the Eocene aragonitic bivalve
Venericardia planicosta and that metabolic factors can exert an important
control. Carpenter & Lohmann (1992) pointed out that high Sr contents
of biotic calcite result from rapid precipitation rates associated with
shell accretion in a variety of Holocene marine phyla (Rhodophyta, Brachiopoda,
Bryozoa, Echinodermata, Coelenterata and Protista) at different locations
and depths and do not exclude the possibility that Mg incorporation may
be a function of the precipitation rate. They also reported on the positive
linear relationship of Sr and Mg contents of Holocene biotic marine calcite.
Busenberg & Plummer (1985), in their study of calcites and selected
aragonites, suggested that the amounts of Na incorporated in calcite vary
as a function of the rate of crystal growth. Mii & Grossman (1994)
found that Mg and Na contents of fossil brachiopod shells correlate inversely
with 18O and pointed out that the Mg, Na concentrations increase with
either increasing temperature or growth rate.
The behaviour of Mn and Fe, minor constituents in seawater, varies inversely
with dissolved oxygen and correlates with nutrients (Kennish & Lute,
1994). Dromgoole & Walter (1990), in their study of calcite overgrowth,
pointed out that Mn and Fe distribution coefficients increase with decreasing
precipitation rates or increasing temperature. The manganese concentration
in neritic benthic organisms also seems to be positively related to slow
growth (Barbin et al., 1991a, b).
In our study, we found a consistent relationship among skeletal Mg/Ca
and Sr/Ca ratios which are inversely related to Fe/Ca, Mn/Ca and Na/Ca
ratios, suggesting that the primary palaeoenvironmental signal could be
partially preserved in the non-luminescent areas of the BID-1 inoceramid
shell (Fig. 6D, E). As temperature and salinity variations in deep-sea
environments are minimal (Tyler, 1988) and light is absent, food (downward
flux of phytodetritus) would appear to be the dominant factor for the
inoceramid life cycle and the best candidate for controlling the variation
of Mg, Sr, Na, Fe and Mn during shell growth. The oscillatory curves of
these elements along intrashell profiles could imply an annual/seasonal
periodicity in inoceramid shell accretion.
The seasonal rain of phytodetritus is common in modern oceans both at
high and low latitudes. Billett et al. (1983), studying the modern sedimentation
of phytoplankton to deep-sea benthos, found a seasonal pulse of detrital
organic material to bathyal and abyssal depths. This organic material
seems to be derived directly from the source, i.e. primary production
in surface-sea, and to sink rapidly to the deep-sea benthos. Considerable
sedimentation of phytodetritus occurs soon after the spring bloom and
continues throughout the early summer in temperate regions. Krantz et
al. (1987), following the work of Heinrich (1962), also noted that in
these regions, phytoplankton blooms occur during the spring as sunlight
intensity increases or occasionally during the autumn in response to the
mixing and nutrient replenishment associated with the deterioration of
the thermocline. Tyler (1988) compiled published data on vertical seasonality
flux in tropical domains. Seasonal flux of organic matter related to surface
production has also been detected in tropical seas (Sargasso Sea, 32°05'
N), and may be associated with the presence of warmer-than-usual water
in the top 500 m of the water column. Two periods of seasonal flux, in
February-March and June-July have been detected at the Panama Basin (~5
°N). The origin of this bimodal flux is related to upwelling periods
(February-March) and phytoplankton blooms (June-July).
The phytodetritus rainfall towards the sea floor along continental margins
probably constituted a seasonally enhanced food source for benthic inoceramid
communities. Therefore, the geochemistry of the alternating dark and clear
growth lines suggests that the growth pattern could be due to the input
of seasonally controlled phytodetritus. If clear lines represent periods
of 'rapid' growth or high mantle metabolic activity during phytodetritus
settling, and dark ones indicate 'slower' growth when organic sedimentation
decays, then a dark-clear line couple may represent one year in the inoceramid
lifetime. But, if we consider two periods of phytodetritus rainfall, one
year in the inoceramid life span is represented by two dark-clear couples.
According to the total number of alternating dark and clear growth lines
observed (≈500), the inoceramid life span could reach about 125
years in the case of two marked periods of organic matter settling, or
250 years, when considering a unique pulse of phytodetritus flux. This
is not an anomalous age, since Turekian et al. (1975) determined by 228Ra
chronology that Tindaria callistiformis, a modern deep-sea clam, reaches
a length of 8.4 mm in about 100 years.
It is well known that the Cretaceous Period, a time of continuous climatic
warmth with ice-free conditions, began to deteriorate towards colder temperatures
during the Campanian-Maastrichtian interval and an increase equator to
pole temperature gradients of 10 to 13°C (~0.4°C/1°) is calculated
(see e.g., Barrera & Savin, 1999) and that the deterioration of the
thermocline, enhanced by periodic changes in the water column, could be
responsible for the seasonal variation in surface productivity. Also,
sudden inputs of strontium resulting from the exposure of continental
shelves and large positive oxygen isotope shifts (~ 1‰), coincident
with dramatic short term sea-level falls, indirectly support the idea
on the feasibility of periodic ice sheets in polar regions during the
Cretaceous Period (Stoll & Schrag, 1996, 2000; Miller et al., 1999).
Sea level falls and continent drift towards high latitudes affect the
land-sea distribution which results in heat transport changes and, if
so, seasonality (Crowley et al., 1986).
At the Bidart section, Clauser (1987), analysing bulk sediment, found
a negative oxygen isotope excursion at the beginning of Member I superimposed
on a period of marine regression. Gómez-Alday et al. (unpublished
data) have found changes in stable isotope composition and clay mineralogy
which affect Member I and Member II in the Sopelana I, Zumaya and Bidart
sections which preclude distinct palaeoclimatic conditions. Inoceramids
are abundant in these members, particularly in Member I, but are poorly
represented at deeper levels and are absent above. This pattern of appearance
indicates that inoceramids could survive under a narrow range of palaeoecological
parameters. If progressive climate deterioration enhanced seasonality,
with slight variations in annual subsurface temperatures and related changes
in surface productivity, the expansion of temporary inoceramid benthic
communities could have been favoured.Conclusions
In the present study, we report that mid-Maastrichtian inoceramid shells
from the Bay of Biscay sections show a lateral westward increase of their
degree of diagenesis, without any substantial textural changes in the
prismatic microstructure. This inference is made on the basis of measurements
of 18O, 13C and minor and trace element concentrations in inoceramid shells
and marl/marly-limestone host-rock samples.
Progressive diagenesis caused a complete luminescent area in the inoceramid
shell MA-33, from the Sopelana I section, and resulted in both an enrichment
of Mn, Fe and Na values as well as a loss of Mg and Sr contents. Nevertheless,
oscillatory curves, primary in origin, are still partially preserved due
to low water/rock ratios (closed system). The loss of Mg is stronger than
that of Sr giving a flattened trend in the luminescent area. In the non-luminescent
shell areas, Mg, Sr, Na, Fe and Mn values are interpreted as being closer
to the original primary values.
The lowest Mn, Fe and Na contents occur in the non-luminescent inoceramid
shell area (BID-1, Bidart section) and Mg and Sr values are well correlated
exhibiting a characteristic trend. This trend suggests that inoceramid
shell accretion probably responded to changes in the calcite precipitation
rate, which is related to seasonally pulsed phytodetritus. We estimate
that the lifetime of some deep-water ocean inoceramid shells could be
about 125 to 250 years. From a palaeoenvironmental point of view, the
inoceramid acme appears to be related to Campanian-Maastrichtian climate
change, which could have imprinted a marked seasonality.Acknowledgements
This study was funded by Research project UPV/EHU 130.310-EB 034/99 financed
by the Universidad del País Vasco. We thank an anonymous reviewer
for constructive comments on an earlier typescript. We would like to thank
Dr D. Merino (Indiana University, Bloomington) and the agency ACTS (ACTS@euskalnet.net)
for improving the English.References
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Fig. 1. Map showing localities referred to in the text, and the areal
extent of Upper Cretaceous strata.
Fig. 2. Stratigraphic logs for the Sopelana I, Zumaya and Bidart cliff
sections, showing the various lithostratigraphic members (I-III), planktonic
foraminiferal zones (Globotruncana gansseri [= Gansserina gansseri] and
Abathomphalus mayaroensis) and the location of inoceramid samples collected
together with the carbonate host-rock.
Fig. 3. Inoceramid shell photomicrographs under SEM (A-D), transmitted
light (E, G) and cathodoluminescence (F, H). In spite of the fact that
the MA-33 inoceramid shell exhibits a well-defined 'honeycomb' microstructure
in the ISL (A), the inter- and intraprism microporosity is not preserved
due to the precipitation of secondary calcite (Sc). This feature contrasts
with that seen in the BID-1 inoceramid shell (C), in which primary porosity
is well preserved (arrows 1 and 2). The same process can be ascribed to
the small prisms of the MA-33 (B) and BID-1 (D) OSL surfaces. When the
degree of secondary calcite precipitation is significant, it can lead
to a marked 18O depletion. This is the case in the Hendaya (HE-2) and
Loya (LO-5) samples (E-H). The HE-2 (E, F) and LO-5 (G, H) inoceramid
samples show interprism zones filled with diagenetic calcite. A fracture
crossing the host rock is observable in the upper part of G. Abbreviations:
Mfr - microfracture; OSL - outer shell layer; MSL - middle shell layer;
ISL - inner shell layer.
Fig. 4. Photomicrograph of inoceramid MA-33 (A); B shows detail of the
MSL, while C is the same microphotograph but modified by image analysis
to increase the contrast of the dark growth lines. Note the regularity
in growth-band thickness.
Fig. 5. Mean carbon and oxygen isotope values from inoceramid shells,
together with host-rock samples, against spatial distribution of the studied
Maastrichtian outcrops. Alpine orogeny folded the series and the ~115
km does not correspond to its original distance.
Fig. 6. MA-33 (A) and BID-1 (B) inoceramid shell CL photomicrographs.
In contrast to that observed in the MA-33 shell (C), in the BID-1 inoceramid
sample (D), two different trends in elemental composition for the non-luminescent
(shaded areas) and luminescent areas (non-shaded areas) were detected.
E shows the relationship among elemental ratios (see text for details).
Optical magnification during chemical analysis limited the accuracy of
spotting, but each spot approximately corresponds to a dark or clear growth
band. Elemental ratios are given in mmol/mol.
Waar zijn de tables ??
Table 1. Mean oxygen and carbon isotope values from host-rock and inoceramid
shells; n = number of samples analysed.
Table 2. Mean elemental ratios along BID-1 and MA-33 shell profiles. CL
= luminescent areas; NCL = non-luminescent areas; n = number of analyses.
Table 3. Comparative values among elemental contents in non-luminescent
zones from the BID-1 inoceramid shell and compiled literature data on
bivalves and other molluscs. |
