[1] An 8 million year record of subtropical and midlatitude shelf-sea temperatures, derived from oxygen isotopes of well-preserved brachiopods from a variety of European sections, demonstrates a long-term Cenomanian temperature rise (16–20°C, midlatitudes) that reached its maximum early in the late Turonian (23°C, midlatitudes). Superimposed on the long-term trend, shelf-sea temperatures vary at shorter timescales in relation to global carbon cycle perturbations. In the mid-Cenomanian and the late Turonian, two minor shelf-sea cooling events (2–3°C) coincide with carbon cycle perturbations and times of high-amplitude sea level falls. Although this evidence supports the hypothesis of potential glacioeustatic effects on Cretaceous sea level, the occurrence of minimum shelf-sea temperatures within transgressive beds argues for regional changes in shelf-sea circulation as the most plausible mechanism. The major carbon cycle event in the latest Cenomanian (oceanic anoxic event 2) is accompanied by a substantial increase in shelf-sea temperatures (4–5°C) that occurred ∼150 kyr after the commencement of the δ13C excursion and is related to the spread of oceanic conditions in western European shelf-sea basins. Our oxygen isotope record and published δ18O data of pristinely preserved foraminifera allow the consideration of North Atlantic surface water properties in the Cenomanian along a transect from the tropics to the midlatitudes. On the basis of fossil-derived δ18O, estimated δw ranges, and modeled salinities, temperature-salinity-density ranges were estimated for tropical, subtropical, and midlatitude surface waters. Accordingly, the Cenomanian temperate shelf-seas waters have potentially the highest surface water density and could have contributed to North Atlantic intermediate to deep waters in the preopening stage of the equatorial Atlantic gateway.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2004PA001015

Midlatitude shelf seas in the Cenomanian‐Turonian greenhouse world: Temperature evolution and North Atlantic circulation

Cathodoluminescence (CL) petrography is a popular tool in investigations of carbonate rocks and their diagenesis The most widespread use of CL in carbonate studies is in cement stratigraphy using zoned cements Visual CL colors and intensities are commonly correlated with analyzed Mn2+ and Fe2+ contents and then used, often in conjunction with other geochemical data (such as stable and radiogenic isotope data, fluid inclusion and paleomagnetic data, etc), to interpret salinity, temperature, and Eh of the paleo-formation waters, and possibly paleofluid flow directions Such applications of CL have great potential for applied research, eg, via establishing porosity evolution in hydrocarbon reservoirs (eg, several articles in Barker and Kopp 1991) Another popular use of CL is to identify marine components (cements or biochems, mainly brachiopods) that are unaltered or least altered by recrystallization, in order to determine the isotopic composition of paleo-ocean water (eg, Popp et al 1986; Lohmann and Walker 1989; Tobin et al 1996)

Application of Cathodoluminescence to Carbonate Diagenesis

This research report contains nine case studies (part II to X) dealing with Palaeozoic and Mesozoic mud mounds, microbial reefs, and modern zones of active micrite production, and two parts (I and XI) summarizing the major questions and results. The formation of different types ofin situ formed micrites (automicrites) in close association with siliceous sponges is documented in Devonian, Carboniferous, Triassic, Jurassic and Cretaceous mounds and suggests a common origin with a modern facies found within reef caves. Processes involved in the formation of autochthonous micrites comprise: (i) calcifying mucus enriched in Asp and Glu, this type presumably is linked to the formation of stromatolites, thrombolites and massive fabrics; (ii) protein-rich substances within confined spaces (e.g. microcavities) result in peloidal pockets, peloidal coatings and peloidal stromatolites, and (iii) decay of sponge soft tissues, presumably enriched with symbiotic bacteria, lead to the micropeloidal preservation of parts of former sponge bodies. As a consequence, there is strong evidence that the primary production of micrite in place represents the initial cause for buildup development. The mode of precipitation corresponds to biologically-induced, matrix-mediated mineralization which results in high-Mg-calcites, isotopically balanced with inorganic cements or equilibrium skeletal carbonates, respectively. If distinct automicritic fabrics are absent, the source or origin of micrite remains questionable. However, the co-occurring identifiable components are inadequate, by quantity and physiology, to explain the enhanced accumulation of fine-grained calcium carbonate. The stromatolite reefs from the Permian Zechstein Basin are regarded as reminiscent of ancestral (Precambrian) reef facies, considered the precursor of automicrite/sponge buildups. Automicrite/sponge buildups represent the basic Phanerozoic reef type. Analogous facies are still present within modern cryptic reef habitats, where the biocalcifying carbonate factory is restricted in space.

Mud mounds: A polygenetic spectrum of fine-grained carbonate buildups

Carbon and oxygen isotopic composition of Silurian brachiopods (Gotland/Sweden): palaeoceanographic implications

Well preserved calcitic belemnite samples from the Basque-Cantabrian basin of northern Spain have been analyzed to improve our current understanding of the chemistry of seawater as an index of paleoceanographic changes during the Early Jurassic, a period punctuated by times of oceanic anoxia and global mass extinctions. Because the oxygen-isotope composition of calcite depends not only on temperature but also on salinity, we present here a combination of Sr/Ca, Mg/Ca, and δ18O analyses that helped us to evaluate both effects separately. Comparison between the temporal records and cross plots of these geochemical parameters shows a good correlation of Mg/Ca with δ18O and a weak correlation of Sr/Ca with δ18O.

Comparison of our belemnite δ18O record with coeval O-isotope profiles from other paleogeographic domains suggests the existence, despite differences in the absolute isotopic values, of a major negative shift during the serpentinus Zone that is regionally reproducible. This event is linked to the Early Toarcian ocean anoxic event (OAE), and suggests the existence of global changes in paleoclimatic and paleoceanographic conditions that are reflected largely in the geochemical composition of seawater. Compared to our results, however, other Early Toarcian δ18O data reported from northern European basins are generally depleted ∼1.3-1.6‰ In contrast, Mg/Ca ratios display similar values. These differences in δ18O data suggest a salinity component for the northern European data and provide evidence for a north-to-south salinity gradient within the north European interior (Boreal) seaway during the Early Jurassic. This study suggests that the secular variation of Mg/Ca is a useful proxy for discriminating between temperature and salinity effects on the oxygen isotope composition of fossil shells, and highlights the prospect of using the Mg/Ca ratios of belemnite calcite to calculate paleotemperatures independently of salinity. The findings of our study may help to calibrate δ18O curves in other basins by removing the salinity component of their δ18O records, which may help to calculate water density gradients between the Jurassic seas of northern and southern Europe.

Elemental and Oxygen Isotope Composition of Early Jurassic Belemnites: Salinity vs. Temperature Signals

Luminescence in calcite and dolomite is governed by physical phenomena that are common to all oxygen-dominated crystalline substances, including other carbonates and silicates. Absorption of excitation energy, energy transfer, and emission involve predictable transitions between electronic energy levels. Strong emission in various colors is always caused by impurities which function as activators of luminescence. Visible luminescence is not expected from pure, undistorted insulators, including carbonates. However, a faint blue 'intrinsic' luminescence, with a broad emission peak (band) around 400 nm, presumably caused by lattice defects, occurs in pure calcite and dolomite, and even in some samples containing impurities. The most important activators in carbonates are transition elements and rare earth elements. Luminescence spectra can be used for activator identification. These spectra are largely independent of the type of excitation, e. g., electron beam (cathodoluminescence = CL), photon (photoluminescence = PL), X-Ray (radioluminescence = RL) excitation, and others. Emission intensities depend on activator, sensitizer, and quencher concentrations, and on the method of excitation. At a given activator concentration, the luminescence intensity generally increases with an increase in excitation energy from PL (relatively weak) to CL (strong). Changes in visual luminescence color between different excitation methods are caused by relative changes in emission peak heights. Mn2* appears to be the most abundant and important activator in natural calcite and dolomite. Substituting for calcium in both minerals, its emission is orange-red to orange-yellow, with a fairly broad band between 570-640 nm (maximum between 590-620 nm). The emission band maximum of Mn2* substituting for Mg2* (in dolomite) is located around 640-680 nm. As little as 10-20 ppm Mn2* in solid solution are sufficient to produce visually detectable luminescence, if total Fe contents are below about 150 ppm. Sm3* activated luminescence can be visually indistinguishable from that activated by Mn2*. The spectrum of Sm3* emission, however, is quite distinct from that of Mn2* and consists of three narrow bands at 562 nm, 604 nm, and 652 nm. Tb3+ and Dy3+ activate green and cream-white luminescence, respectively. The main emission of Tb3* is at 546 nm. The emission of Dy3t consists of three bands, located at 484 nm, 578 nm, and 670 nm. Emission from Eu-containing calcite is red or blue. Narrow spectral bands of 590 nm, 614 nm, and 656 nm are caused by Eu3* and correspond to the red emission. A broad emission spanning a large range of shorter wavelengths is caused by Eu2* and corresponds to the blue emission. As in the case of Sm3*-activated luminescence, the red Eu3* luminescence can be mistaken visually for Mn2*-activated luminescence. Visual luminescence detection limits for rare earths are on the order of 10 ppm. Pb2* is an activator, with an emission band around 480 nm, but it also is a sensitizer of Mn2*-activated luminescence in carbonates. Another recognized sensitizer for Mn2* in carbonates is Ce3*. Sensitizers appear to be effective at concentrations as low as 10 ppm in calcite. Quenchers of Mn2*-activated luminescence in carbonates are Fe2*, Co2*, Ni2*, and Fe3*. The concentrations at which quenchers appear to be effective may vary from element to element and with host mineralogy. Effective minimum concentrations as low as 30-35 ppm have been reported for calcite. The interplay of Mn2* and Fe2*, commonly regarded to be the most important activator and quencher, respectively, in determining the luminescence characteristics of natural carbonates is not well understood because the available data are partially inconsistent. The Mn/Fe ratio may exert a control on luminescence intensity. Mn and Fe concentrations at which 'bright' CL changes to 'dull' can be determined only semi-quantitatively. The available data on the concentration of Mn2* at which quenching starts are partially inconsistent Consequently, the Mn2* concentration at which concentration extinction occurs has not been determined unequivocally. The data presented and summarized in this paper can be used as a basis for the interpretation of luminescence of geological materials. Li particular, knowledge of the possibilities and complexities of activation, sensitization, and quenching has great potential for the interpretation of diagenetic carbonate cements.

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Roger A. Mason

Papers

Causes and Emission of Luminescence in Calcite and Dolomite