Palaeogeography, Palaeoclimatology, Palaeoecology
Early Maastrichtian stable isotopes: changing deep water sources in the North Atlantic?
Introduction
A global cooling during the Campanian and Maastrichtian has been suggested based on planktic and benthic foraminiferal stable isotope data (e.g., Clarke and Jenkyns, 1999, Huber et al., 2002). This cooling marks the onset of waning greenhouse conditions in the Cretaceous. It coincides with three second-order regressions (Haq et al., 1987) that were possibly caused by ice sheet formation in Antarctica (e.g., Miller et al., 1999, Miller et al., 2003) and influenced the carbon cycle and diversification of marine biota (e.g., Li and Keller, 1998a, Frank and Arthur, 1999).
The Campanian/Maastrichtian cooling is thought to have had significant impact on the formation of deep water masses in the world's oceans (e.g., MacLeod and Huber, 1996, Barrera and Savin, 1999). Under strong greenhouse conditions such as those of the mid-Cretaceous and Paleogene, oceanic circulation was predominantly driven by the sinking of warm, saline waters in low-latitude regions with excessive evaporation (e.g., Brass et al., 1982, Kennett and Stott, 1990, Kennett and Stott, 1991, Johnson et al., 1996, Bice et al., 1997). This circulation mode is thought to have prevailed even during the cooler Late Cretaceous (Prentice and Matthews, 1988). In addition to the low latitudes, the southern and northern high latitudes of the Pacific Ocean may have served as additional sites of bottom water formation. This view is based on general circulation models (Barron and Peterson, 1990, Poulsen et al., 2001), depositional patterns of hydrothermally precipitated phases (Dickens and Owen, 1995), and stable isotopic data (Li and Keller, 1999).
During the Campanian to Maastrichtian cooling period, intermediate to deep water may have started to form in the Atlantic Ocean (e.g., Saltzman and Barron, 1982, Barrera and Savin, 1999, D'Hondt and Arthur, 2002). More specifically, intermediate to deep water sources during this period may have been located in the high-latitude South Atlantic (Barrera and Huber, 1990, Barrera et al., 1997) and in the North Atlantic (Corfield and Norris, 1996, Frank and Arthur, 1999). However, based on comparison of benthic foraminiferal isotopes, Frank and Arthur (1999) proposed a North Atlantic that was separated from other ocean basins during the Early Maastrichtian. Therefore, deep waters from a high-latitude South Atlantic source could not affect North Atlantic deep-water masses. A major reorganization of the oceanic circulation with a shift from low-latitude to high-latitude deep water sources, probably occurred around the Early/Late Maastrichtian boundary (MacLeod and Huber, 1996), resulting in a modern circulation system.
As is known from the Quaternary (e.g., Oppo and Fairbanks, 1990, Lehmann and Keigwin, 1992, LeGrand and Wunsch, 1995, Ishman, 1996, deMenocal et al., 1997), reorganization in the mode and intensity of deep water formation must have had a significant influence on climate. During the Quaternary, short-term changes (hundreds of kiloyears) in deep water sources occurred. During full interglacials, the production of lower North Atlantic deep water (LNADW) occurred predominantly in the Norwegian and Greenland Seas. During glacials, upper North Atlantic deep water (UNADW) was mainly produced in the Mediterranean and Labrador Seas (Oppo and Fairbanks, 1990). Reduced sea surface temperatures in the North Atlantic during the Last Glacial Maximum in comparison to the Holocene favored the production of UNADW over the production of LNADW (Boyle and Keigwin, 1987).
To date, studies on short-term changes in intermediate to deep water sources during the reorganization of the circulation mode in the Late Cretaceous have been performed by Barrera et al. (1997) and Li and Keller (1998b). Barrera et al. (1997) demonstrated that the thermohaline circulation in the Early Maastrichtian (71–70 Ma) was marked by rapid reversals in the dominant mode and direction of deep water formation. Based on high-resolution stable isotope data, Li and Keller (1998b) suggested a change from a higher-latitude intermediate water source to a lower-latitude warm, saline, dense water source for at least 300 ky during the latest Cretaceous.
However, as these studies focussed on ODP and DSDP cores from the South Atlantic, there is still a lack of high-resolution studies on longer time intervals from the Campanian/Maastrichtian North Atlantic. Therefore, we measured stable oxygen and carbon isotopes from monospecific planktic and benthic foraminifera from Blake Nose (DSDP Site 390A; North Atlantic; Fig. 1) to determine the role of short-term changes between different intermediate to deep water sources for the Early Maastrichtian North Atlantic.
Section snippets
Geological setting
Site 390A was drilled during DSDP Leg 74 at Blake Nose, a gently sloping topographic high that extends into the western Atlantic from Blake Plateau (30°08.54′N; 76°06.74′W). It was drilled in a water depth of 2665 m (Shipboard Scientific Party, 1978). Cretaceous sediments at Site 390A are composed of nannofossil ooze and represent the uppermost Campanian and Maastrichtian (Shipboard Scientific Party, 1978).
During the Late Cretaceous, Blake Nose was located at a paleolatitude of ∼30° North (Hay
Material and methods
We analyzed 133 samples of monospecific planktic (Contusotruncana fornicata) and 98 samples of monospecific benthic foraminifera (Gavelinella beccariiformis and Nutallides truempyi) from DSDP Site 390A (Blake Nose, North Atlantic). To minimize isotopic variability, we picked specimens from narrow size fractions (250–315 μm for planktic foraminifera and 125–250 μm for benthic foraminifera). Foraminiferal tests generally have a good to very good preservation showing no evidence of significant
Chronostratigraphic framework
Biostratigraphically, the studied section at DSDP Site 390A (cores 13-2 to 14-5) comprises the lower part of the calcareous nannoplankton Zone NC21 and the Globotruncana falsostuarti–Gansserina gansseri Planktic Foraminiferal Zone of the lowermost Maastrichtian (Roth, 1978, MacLeod et al., 2000, MacLeod and Huber, 2001). Above the studied interval, a significant hiatus cuts out much of the middle Maastrichtian sequence (MacLeod et al., 2000), including the lower part of the Abathomphalus
Carbon isotopes
In the lower part of the investigated interval (71.3–70.7 Ma) the planktic foraminiferal δ13C record is characterized by values around 2.1‰ (Fig. 2; Table 1). Between 70.7 and 70.2 Ma, an increase of up to 0.7‰ could be recognized (from 2.1‰ to 2.8‰), followed by δ13C fluctuations between 2.4‰ and 2.6‰ in the upper part of the studied section. The uppermost part (<69.7 Ma) shows a decrease of 0.3‰. The benthic foraminiferal carbon isotope values show a higher variability than the planktic ones.
Discussion
The Late Cretaceous succession at DSDP Site 390A was deposited at a paleo-water depth of 1000–2000 m (Norris et al., 1998), an interval that in modern oceans is bathed in intermediate to deep waters. Thus, the benthic foraminiferal stable isotope values of our study yield a characterization of intermediate to deep waters.
Frank and Arthur (1999) have noted that Early Maastrichtian intermediate to deep water benthic foraminiferal carbon isotope records from the tropical Pacific, southern Indian
Proposed model for deep water changes
The proposed deep water changes as indicated by benthic foraminiferal stable isotopes occurred at an average cyclicity of ∼104 ky (Fig. 2). The spectral analysis of grey values exhibits a cycle of 118 ky as the most significant in the investigated succession (Fig. 3). As both these values are relatively close to orbital forcing within the eccentricity band (Berger et al., 1992), they probably indicate eccentricity-controlled forcing on the proposed fluctuations in the intensity of high- and
Glacioeustacy during the Early Maastrichtian
The general cooling trend during the Campanian/Maastrichtian is punctuated by several short-term cooling events, with the strongest event between 71 and 70 Ma (Barrera and Savin, 1999). During that time, planktic δ18O values increased by 0.7‰ in the South Atlantic, indicating a cooling of up to 2.5 °C in the surface waters (e.g., Li and Keller, 1998b). Recently, this temperature decrease in combination with sequence stratigraphic data from New Jersey has been interpreted to reflect ice sheet
Conclusions
Our high-resolution stable carbon and oxygen isotope data from Blake Nose suggest high-frequency intermediate to deep water changes in the Early Maastrichtian. Possible sources for these waters are in the low-latitude eastern Tethys and the high-latitude North Atlantic. We propose that eccentricity-steered changes in insolation controlled the origin and composition of water masses bathing Blake Nose during the Early Maastrichtian. High insolation led to warming and diminished deep water
Acknowledgments
We thank the Deep Sea Drilling Project for providing samples used in this study. T. Frank, K. Miller, and J. Pross are thanked for fruitful comments and discussions. We thank B.T. Huber, M.B. Hart, and an anonymous reviewer for their helpful and constructive reviews. We are grateful to H. Erlenkeuser for stable isotope measurements. Funding for this study was provided by the German Research Foundation (grant He 697/41-1/2).
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Present address: Southampton Oceanography Centre, School of Ocean and Earth Science, European Way, Southampton SO14 3ZH, UK.