Oliver Friedrich

Evolution of middle to Late Cretaceous oceans—A 55 m.y. record of Earth's temperature and carbon cycle

A new 55 m.y. global compilation of benthic foraminifera δ 13 C and δ 18 O for the middle to Late Cretaceous shows that there was widespread formation of bottom waters with temperatures >20 °C during the Cretaceous greenhouse world. These bottom waters filled the silled North Atlantic and probably originated as thermocline or intermediate waters in the tropical oceans. Carbon burial during the Cretaceous oceanic anoxic events produced a positive δ 13 C shift in global carbon reservoirs, but this is not particularly large, especially by comparison with the remarkable Late Paleocene carbon maximum. The interbasin δ 13 C gradient was unusually large during the Cretaceous hot greenhouse, probably because the North Atlantic sills prevented the free exchange of waters in the deep basin. The hot greenhouse ended when the Equatorial Atlantic Gateway opened sufficiently to flood the deep North Atlantic with relatively cool polar waters formed in the Southern Ocean.

Isotopic evidence for glaciation during the Cretaceous supergreenhouse

The Turonian (93.5 to 89.3 million years ago) was one of the warmest periods of the Phanerozoic eon, with tropical sea surface temperatures over 35°C. High-amplitude sea-level changes and positive δ18O excursions in marine limestones suggest that glaciation events may have punctuated this episode of extreme warmth. New δ18O data from the tropical Atlantic show synchronous shifts ∼91.2 million years ago for both the surface and deep ocean that are consistent with an approximately 200,000-year period of glaciation, with ice sheets of about half the size of the modern Antarctic ice cap. Even the prevailing supergreenhouse climate was not a barrier to the formation of large ice sheets, calling into question the common assumption that the poles were always ice-free during past periods of intense global warming.

Forcing mechanisms for mid-Cretaceous black shale formation: evidence from the Upper Aptian and Lower Albian of the Vocontian Basin (SE France)

Calcareous nannoplankton, palynomorph, benthic foraminifera, and oxygen isotope records from the supraregionally distributed Niveau Paquier (Early Albian age, Oceanic Anoxic Event 1b) and regionally distributed Niveau Kilian (Late Aptian age) black shales in the Vocontian Basin (SE France) exhibit variations that reflect paleoclimatic and paleoceanographic changes in the mid-Cretaceous low latitudes. To quantify surface water productivity and temperature changes, nutrient and temperature indices based on calcareous nannofossils were developed. The nutrient index strongly varies in the precessional band, whereas variations of the temperature index reflect eccentricity. Since polar ice caps were not present during the mid-Cretaceous, these variations probably result from feedback mechanisms within a monsoonal climate system of the mid-Cretaceous low latitudes involving warm/humid and cool/dry cycles. A model is proposed that explains the formation of mid-Cretaceous black shales through monsoonally driven changes in temperature and evaporation/precipitation patterns. The Lower Albian Niveau Paquier, which has a supraregional distribution, formed under extremely warm and humid conditions when monsoonal intensity was strongest. Bottom water ventilation in the Vocontian Basin was diminished, probably due to increased precipitation and reduced evaporation in regions of deep water formation at low latitudes. Surface water productivity in the Vocontian Basin was controlled by the strength of monsoonal winds. The Upper Aptian Niveau Kilian, which has a regional distribution only, formed under a less warm and humid climate than the Niveau Paquier. Low-latitude deep water formation was reduced to a lesser extent and/or on regional scale only. The threshold for the formation of a supraregional black shale was not reached. The intensity of increases in temperature and humidity controlled whether black shales developed on a regional or supraregional scale. At least in the Vocontian Basin, the increased preservation of organic matter at the sea floor was more significant in black shale formation than the role of enhanced productivity.

High-resolution carbon isotope records of the Aptian to Lower Albian from SE France and the Mazagan Plateau (DSDP Site 545): a stratigraphic tool for paleoceanographic and paleobiologic reconstruction

High-resolution carbon isotope stratigraphy is established for the Aptian to Lower Albian of the Vocontian Basin (SE France), and correlated to the carbon isotope record of the Mazagan Plateau (DSDP Site 545). The carbon isotope stratigraphy of the Vocontian Basin is proposed as a standard reference curve for the Aptian to Lower Albian, due to the completeness and high temporal resolution of the stratigraphic succession, the good biostratigraphical time control, and the frequent occurrence of regional to global black shale horizons including Oceanic Anoxic Events 1a (OAE 1a) of the Lower Aptian and OAE 1b of the Lower Albian. The carbon isotope record appears better suited for long-distance short-term correlation of different marine and terrestrial environments than biostratigraphy because of the synchroneity of carbon isotope signals in a range of sediment types. However, the combination of both biostratigraphy and carbon isotope stratigraphy provides an effective tool to reconstruct biotic change and paleoceanography, and to correlate regional to global black shale horizons in different marine environments. This combined approach allows us to ascertain the synchroneities or diachroneities of first and last appearances of biostratigraphic marker species. Based on the demonstrated diachroneity of important biostratigraphic markers of the Aptian/Albian boundary, the globally observed break point between the end of the uppermost Aptian positive carbon isotope excursion and the onset of the pronounced negative shift of δ13C values, is an alternative criterion. The distinctive structure and amplitudes of the carbon isotope record are observed in both the inorganic and organic carbon and can therefore be recognized in all marine and terrestrial environments of the Aptian to Lower Albian.

Stable organic carbon isotope stratigraphy across Oceanic Anoxic Event 2 of Demerara Rise, western tropical Atlantic

Ocean Drilling Program (ODP) Leg 207 recovered expanded sections of organic-carbon-rich laminated shales on Demerara Rise (western tropical Atlantic). High-resolution organic carbon isotope and total organic carbon (TOC) records are presented, which span the Cenomanian-Turonian boundary interval (CTBI), including the Oceanic Anoxic Event (OAE) 2, from four sites oriented along a NW striking depth transect. These records represent the first high-resolution carbon isotope records across OAE 2 from the South American margin of the tropical Atlantic. Due to the scarcity of age significant fossils, the main purpose of this study was to develop a detailed carbon isotope stratigraphy in order to correlate the CTBI across the depth transect and to tie this to biostratigraphically well-defined sections in the Western Interior Basin (Pueblo, USA), boreal shelf seas (Eastbourne, England), and western Tethys (Oued Mellegue, Tunisia). All four sections studied document a 6‰ increase of ?13Corg values at the base of the CTBI, which is followed by an interval of elevated ?13Corg values and a subsequent decrease. Our results supply an important stratigraphic base for subsequent paleoceanographic studies on Late Cenomanian to Early Turonian sediments from Demerara Rise and elsewhere.

Testing for ice sheets during the mid-Cretaceous greenhouse using glassy foraminiferal calcite from the mid-Cenomanian tropics on Demerara Rise

The mid-Cretaceous is widely considered the archetypal ice-free greenhouse interval in Earth history, with a thermal maximum around Cenomanian-Turonian boundary time (ca. 90 Ma). However, contemporaneous glaciations have been hypothesized based on sequence stratigraphic evidence for rapid sea-level oscillation and oxygen isotope excursions in records generated from carbonates of questionable preservation and/or of low resolution. We present new oxygen isotope records for the mid-Cenomanian Demerara Rise that are of much higher resolution than previously available, taken from both planktic and benthic foraminifers, and utilizing only extremely well preserved glassy foraminifers. Our records show no evidence of glaciation, calling into question the hypothesized ice sheets and rendering the origin of inferred rapid sea-level oscillations enigmatic. Simple mass-balance calculations demonstrate that this Cretaceous sea-level paradox is unlikely to be explained by hidden ice sheets existing below the limit of δ 18 O detection.

Early Maastrichtian carbon cycle perturbation and cooling event: Implications from the South Atlantic Ocean

Published stable isotope records in marine carbonate are characterized by a positive ?18O excursion associated with a negative ?13C shift during the early Maastrichtian. However, the cause and even the precise timing of these excursions remain uncertain. We have generated high-resolution foraminiferal stable isotope and grey scale records for the latest Campanian to early Maastrichtian (~73-68 Ma) at two Ocean Drilling Program sites, 525 (Walvis Ridge) and 690 (Weddell Sea). We demonstrate that the negative ?13C excursion is decoupled from the ?18O increase with a lag of about 600 kyr. Our ?13C records (both planktic and benthic) show an amplitude for the negative excursion of 0.7 ‰ that falls between about 72.1 and 70.7 Ma. Our planktic ?18O records indicate an overall increase of 1.2 ‰ from 73 to 68 Ma at Site 690, whereas at Site 525 they record a slightly smaller increase (~1 ‰) that peaks around 70.1 Ma with decreasing values thereafter. Our benthic ?18O data indicate an increase of ~1.5 ‰ at Site 525 and ~0.7 ‰ at Site 690 between about 71.4 and 69.9 Ma. Benthic ?18O values show different baseline values at the two sites before and after the excursion but the larger increase at Site 525 means that the values attained at the peak of the excursion are similar at the two sites. We interpret this observation in terms of water mass changes. The excursion is interpreted to reflect a cooling of bottom waters in response to the strengthening contribution of intermediate- to deep-water production in the high southern latitudes rather than increased ice volume. The associated carbon-cycle perturbations that we observe are interpreted to reflect a weakening of surface-water stratification and increased productivity, as supported by our grey value data.

Formation of the Late Aptian Niveau Fallot black shales in the Vocontian Basin (SE France): evidence from foraminifera, palynomorphs, and stable isotopes

Abstract We present paleoceanographic models for the formation of the marlstone facies and the most prominent black shale intervals of the Late Aptian Niveau Fallot black shale succession from the Vocontian Basin (SE France). In the lower part of the succession, the composition of benthic foraminiferal assemblages of the Niveau Fallot 2′ and 2″ black shales suggests an enhanced burial of organic matter due to more eutrophic conditions and resulting low oxygen conditions at the seafloor. In the upper part of the succession (including Niveau Fallot 3 and 4), a third-order sea-level fall, indicated by a decrease in the stable carbon isotope values, may have resulted in a reduced water mass exchange between the Vocontian Basin and the western Tethyan Ocean. This may have led to dysoxic conditions at the seafloor of the Vocontian Basin and may have favored the formation of organic-rich sediments as shown by benthic foraminiferal assemblages. Decreased evaporation, however, is proposed as the most important mechanism causing a restriction of deep water formation within the basin which finally led to the formation of the Niveau Fallot 3 and 4 black shales. Based on our investigations, the formation of the Niveau Fallot black shales was caused by different factors, the most prominent ones include sea-level fluctuations, increasing productivity, and changes in precipitation and evaporation rates. Furthermore, Niveau Fallot 3 can probably be correlated with the Thalmann black shale event in California and a dark horizon at the Mazagan Plateau (DSDP Site 545).

Campanian – Maastrichtian carbon isotope stratigraphy: shelf-ocean correlation between the European shelf sea and the tropical Pacific Ocean

The long-term climate cooling during Campanian - Maastrichtian times is not well understood to date, especially because of the uncertainty introduced by low temporal resolution of biostratigraphy and the pronounced provincialism between tropical and temperate taxa. Two new high-resolution carbon isotope records derived from the boreal shelf-sea section at Lagerdorf-Kronsmoor-Hemmoor, northern Germany and the tropical Pacific at Deep Sea Drilling Project Site 305, Shatsky Rise, reduce these uncertainties. The records can be correlated with an accuracy not achieved by biostratigraphic methods so far. Distinct carbon isotope events in the late Campanian and the early Maastrichtian can be identified at both localities suggesting to represent global carbon cycle perturbations. Especially, the negative carbon isotope excursion in the early Maastrichtian, a pronounced feature of open-ocean records from the Pacific and Southern oceans, is recognized for the first time at a shelf-sea locality related to the North Atlantic Ocean. Furthermore, two short-term positive excursions are identified as superimposed signals to this event. The improved stratigraphy provides the unique opportunity to recognize leads and lags between the carbon cycle and ocean circulation of different marine settings and ecosystems, leading to a better understanding of their causes and effects.

CLIMATIC CHANGES IN THE LATE CAMPANIAN—EARLY MAASTRICHTIAN: MICROPALEONTOLOGICAL AND STABLE ISOTOPIC EVIDENCE FROM AN EPICONTINENTAL SEA

Benthic foraminiferal and calcareous nannofossil assemblages, as well as stable isotope data from the Campanian/Maastrichtian boundary interval (;71.4 to ;70.7 Ma) of the Kronsmoor section (North German Basin), were investigated in order to characterize changes in surface-water productivity and oxygen content at the seafloor and their link to climatic and paleoceanographic changes. A nutrient index based on calcareous nannofossils is derived for the high-latitude, epicontinental North German Basin, reflecting changes in surface-water productivity. Oxygen isotopes of well-preserved planktic foraminiferal specimens of Heterohelix globulosa reflect warmer surface-water temperatures in the lower part of the studied succession and a cooling of up to 28C (0.5‰) in the upper part (after 71.1 Ma). For the lower and warmer part of the investigated succession, benthic foraminiferal assemblages and the calcareous nannofossils indicate well-oxygenated bottom waters and low-surface water productivity. In contrast, the upper part of the succession is characterized by cooler conditions, lower oxygen content at the seafloor and increasing surfacewater productivity. It is proposed that the cooling phase starting at 71.1 Ma was accompanied by increasing surface-water mixing caused by westerly winds. As a consequence of mixing, nutrients were advected from subsurface waters into the mixed layer, resulting in increased surface-water productivity and enhanced organic matter flux to the seafloor. We hypothesize that global sea-level fall during the earliest Maastrichtian ( ;71.3 Ma), indicated by decreasing carbon isotope values, may have led to a weaker water mass exchange through narrower gateways between the Boreal Realm and the open North Atlantic and Tethys oceans. Both the weaker water mass exchange and enhanced surface-water productivity may have led to slightly less ventilated bottom waters of the upper part of the studied section. Our micropaleontological and stable isotopic approach indicates short-term ( ,100 kyr) changes in oxygen consumption at the seafloor and surface-water productivity across the homogeneous Boreal White Chalk succession of the North German Basin.