National Academies Press: OpenBook

Effects of Past Global Change on Life (1995)


Suggested Citation:"ASSOCIATION BETWEEN MASS EXTINCTION AND OCEANIC WARMING." National Research Council. 1995. Effects of Past Global Change on Life. Washington, DC: The National Academies Press. doi: 10.17226/4762.
Page 97
Suggested Citation:"ASSOCIATION BETWEEN MASS EXTINCTION AND OCEANIC WARMING." National Research Council. 1995. Effects of Past Global Change on Life. Washington, DC: The National Academies Press. doi: 10.17226/4762.
Page 98
Suggested Citation:"ASSOCIATION BETWEEN MASS EXTINCTION AND OCEANIC WARMING." National Research Council. 1995. Effects of Past Global Change on Life. Washington, DC: The National Academies Press. doi: 10.17226/4762.
Page 99
Suggested Citation:"ASSOCIATION BETWEEN MASS EXTINCTION AND OCEANIC WARMING." National Research Council. 1995. Effects of Past Global Change on Life. Washington, DC: The National Academies Press. doi: 10.17226/4762.
Page 100

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TERMINAL PALEOCENE MASS EXTINCTION IN THE DEEP SEA: ASSOCIATION WITH GLOBAL WARMING 97 phy (Kennett and Stott, 1991; Thomas, 1992) have strengthened the concept that the extinction was synchronous throughout the oceans. Nevertheless, this still requires confirmation with high-resolution studies at numerous sequences that can be accurately correlated by using carbon isotopic, paleomagnetic, and biostratigraphic data (Sinha and Stott, 1994). Immediately following the extinction, the benthic foraminiferal assemblages were dominated by small, thin-walled specimens (Thomas, 1990). Benthic taxa that survived the extinction included Nuttallides truempyi, which became a dominant component in the Eocene, as well as Bulimina semicostata and other taxa making up what has been termed the Nuttallides truempyi assemblage. This new, relatively low-diversity assemblage includes about six forms that dominated Early to Middle Eocene benthic foraminiferal assemblages (Tjalsma and Lohmann, 1983; Miller et al., 1987). Faunal assemblages following the extinction are less cosmopolitan (Thomas, 1990). Although the extinction event was abrupt, there is some evidence that the S. beccariformis assemblage became progressively restricted to shallower depths during the Paleocene (Tjalsma and Lohmann, 1983; Miller et al., 1987). Also, the relative abundances of certain forms in this assemblage decreased during the Late Paleocene and were replaced by forms more typical of the Nuttallides truempyi assemblage of latest Paleocene to Early Eocene age (Miller et al., 1987). These changes culminated at the mass extinction and suggest that some form of biological threshold was surpassed. For several million years following the extinction there occurred a radiation of benthic foraminiferal taxa. These probably filled vacancies left by the latest Paleocene extinctions (Tjalsma and Lohmann, 1983; Miller et al., 1987). The postextinction assemblages included long-ranging forms such as Pullenia bulloides and Globocassidulina subglobosa (Thomas, 1990). The radiation caused a diversity increase that peaked during the early Middle Eocene. Nevertheless, the high diversity values of the Late Cretaceous and Early Paleocene were never attained again (Thomas, 1990). In contrast to the benthic assemblages, oceanic planktonic microfossil assemblages underwent no mass extinction at the end of the Paleocene, but did exhibit distinct change in the species composition in the Antarctic. A general increase in diversity marks the Late Paleocene high- to middle-latitude assemblages of planktonic foraminifera, calcareous nannofossils, and dinoflagellates (Premoli-Silva and Boersma, 1984; Oberhänsli and Hsü, 1986; Stott and Kennett, 1990; Pospichal and Wise, 1990). This increase in diversity stemmed, in part, from the incursion of lower-latitude groups into the Southern Ocean. The diversity increase at the end of the Early Paleocene was superimposed on a longer-term increase that began during the Paleocene, following the K/T boundary extinctions (Corfield, 1987). The plankton diversity increase may have been caused by the increased surface water temperatures at high- to middle-latitude regions. This increase in surface water temperatures was particularly pronounced in the Antarctic during the latest Paleocene, as reflected by the relatively brief appearance of the subtropical-tropical morozovellid group and a peak in discoaster abundance (Pospichal and Wise, 1990; Stott and Kennett, 1990). The emigration of these warm-loving planktonic microfossils to the Antarctic was particularly pronounced during the mass extinction. In one Antarctic site 32% of the planktonic foraminiferal species appeared for the first time in the latest Paleocene, 27% underwent major abundance changes, and only 13% were eliminated from the assemblages (Lu and Keller, 1993). Most new entries were surface dwellers. Of those that were eliminated, most were deeper dwellers such as the subbotinids (Lu and Keller, 1993). Coeval low latitude planktonic assemblages underwent little change (Miller et al., 1987; Miller, 1991) presumably because of the relatively stable sea surface temperatures (Stott, 1992). ASSOCIATION BETWEEN MASS EXTINCTION AND OCEANIC WARMING In earlier work (Kennett and Stott, 1990; Stott et al., 1990) we discovered a dramatic negative oxygen and carbon isotopic excursion of brief duration (Figure 5.1) that coincided closely with the terminal Paleocene benthic foraminiferal extinction event in an Antarctic Paleogene sequence (ODP Site 690B) (Thomas 1989, 1990). This discovery stimulated a high-resolution study of the extinction event (Kennett and Stott, 1991). Results from that study demonstrated the intimate temporal relationship between the mass extinction and a large oxygen and carbon isotope excursion in both benthic and planktonic foraminifera. Planktonic values of δ18O abruptly decreased by 1.0 to 1.5%o, and by about 2%o in the benthics; values of δ13C also decreased by 4%o in surface-dwelling planktonic foraminifera, and -2%o in the deeper-dwelling planktonic and benthic forms. The planktonic foraminifer Acarinina praepentacamerata records the lowest oxygen and highest carbon isotope values within the excursion, consistent with an inferred near-surface habitat (Stott et al., 1990; Kennett and Stott, 1991). The species of Subbotina record higher δ18O and lower δ13C values, indicating a deep water planktonic habitat and/or a preference for cooler months of the year (Stott et al., 1990; Kennett and Stott, 1991). The highest δ18O and lowest δ13C values are exhibited by the benthic foraminifer Nuttalides truempyi, reflecting its habitat in relatively nutrient- rich high latitude deep water.

TERMINAL PALEOCENE MASS EXTINCTION IN THE DEEP SEA: ASSOCIATION WITH GLOBAL WARMING 98 Figure 5.1 Composite oxygen isotopic record of planktonic foraminifers of ODP Sites 689 and 690 in the Antarctic Ocean (from Stott et al., 1990). Note the abrupt negative excursion near the Paleocene-Eocene boundary. Time scale follows that of Berrgren et al. (1985), although the Paleocene-Eocene boundary is now placed at 55 Ma (Cande and Kent, 1992). The temperature scale is based on the assumption of no significant ice sheet prior to the Early Oligocene and does not account for variation in surface water salinity. During the latest Paleocene interval immediately preceding the extinction (before ~55.33 Ma), surface water temperatures estimated from oxygen isotopic values were -13 to 14°C (Figure 5.2). Deep water temperatures were -10°C at about 2100 m. Thus, before the extinction, little temperature difference existed between surface and deep water in the Antarctic. The excursion began abruptly at 55.33 Ma, with conspicuous decreases in δ18O and δ13C values, followed by a return to values only slightly lower than those before the

Figure 5.2 Changes in oxygen and carbon isotopic composition of planktonic foraminifera (Acarinina praepentacamerata and Subbotina) and a benthic foraminifer (Nuttalides TERMINAL PALEOCENE MASS EXTINCTION IN THE DEEP SEA: ASSOCIATION WITH GLOBAL WARMING truempyi) in the latest Paleocene (55.0 to 55.6 Ma) in relation to mass extinction in benthic foraminifera in ODP Site 690B, Maud Rise, Antarctica. These changes constitute the terminal Paleocene isotopic excursion. Note the abrupt negative isotopic shifts coinciding with the extinction. (Figure modified after Kennett and Stott, 1991.) 99

TERMINAL PALEOCENE MASS EXTINCTION IN THE DEEP SEA: ASSOCIATION WITH GLOBAL WARMING 100 excursion (Figure 5.2). The δ18O and δ13C changes are reflected in all three foraminiferal taxa, although at different amplitudes (Figure 5.2). The significance of these differences is discussed by Kennett and Stott (1991). We limit our discussion here to the trends of critical paleoenvironmental importance. The largest δ18O shift (2.0%o) at the beginning of the excursion is recorded by the benthic foraminifera, an intermediate shift (1.5%o) by deeper-dwelling planktonics, and the smallest shift (1.0%o) by shallow-dwelling planktonic forms (Figure 5.2). The initial oxygen isotopic shift exhibited by the surface-dwelling form, which coincided with the mass extinction, possibly reflects an increase in surface water temperatures from 14 to 18°C. This was followed by an additional δ18O decrease of -1.0%o, indicating a further possible temperature increase in surface waters to 22°C. The brief interval represented by the excursion was likely the warmest of the entire Cenozoic although, as discussed later, some fraction of the decrease in δ18O values may have resulted from reduction in surface water salinity. Of great significance, however, is the observation that the largest δ18O change was recorded by the benthic, rather than the planktonic, foraminifera (Figure 5.2). Thus, deep waters warmed more than surface waters. For a brief interval, beginning at -55.31 Ma, deep waters had warmed to such a degree that the temperature gradient between deep and surface waters was virtually eliminated at this location in the Antarctic region. The extinctions occurred at the beginning of the temperature excursion. The initial, rapid temperature rise encompassed -3000 yr and was followed by a more gradual decrease in ocean temperatures at all water depths. At the end of the excursion, the water column in this Antarctic region was only slightly warmer than it had been immediately before the excursion less than 100,000 yr earlier. The magnitude of carbon isotopic change between the planktonic and benthic foraminfera was different from that of oxygen isotopes. Whereas the benthic (bottom dwellers) recorded the largest δ18O change, it was the plankton that recorded the largest δ13C change (4%o). The 4%o shift in δ13C is the largest so far known for the Cenozoic Period. The magnitude of the shift clearly underscores the significance of this event. During the brief interval at -55.32 Ma when the vertical δ18O gradient was eliminated, the previously large surface to deep water δ13C gradient was also almost completely eliminated. The cause of the δ13C change remains enigmatic. However, Stott (1992) presented evidence that the δ13C of marine organic matter became more positive at the time of the excursion. If this was a global phenomenon, it would suggest that the negative δ13C excursion recorded in foraminiferal calcite resulted from a redistribution of δ12C between photosynthetic organic matter and the inorganic pool of carbon in the Late Paleocene oceans. It is clear from this Antarctic record that the mass extinction coincided with the beginning of the sharp, negative shifts in δ18O and δ13C. However, to determine whether all the extinctions occurred simultaneously and in conjunction with the initiation of the δδO and δ13C change, Kennett and Stott (1991) increased the sample resolution to only 1-cm intervals (-800 yr) across the extinction interval in Site 690B (Figure 5.3). This was possible because the interval was not bioturbated (Kennett and Stott, 1991). Before the extinction, benthic foraminiferal assemblages (>150-µm fraction) were diverse, averaging about 60 species, or even more (Thomas, 1990). Assemblages included an abundance of forms interpreted to be of both infaunal and epifaunal habit (Corliss and Chen, 1988; Thomas, 1990). This included a high diversity of trochospiral and other coiled forms. The extinction in Site 690B (Figure 5.3) involved a rapid drop in benthic foraminiferal diversity (>150 µm) from ~60 to 17 species, representing a diversity reduction of 72% within 3000 yr (4 cm). Many distinct taxa such as Stensioina beccariiformis and Neoflabellina disappeared early, during an interval of less than 1500 yr (Figure 5.3). Most trochospiral forms such as Stensioina had disappeared by the midpoint of the oxygen isotopic shift. The survival of a higher proportion of infaunal forms indicates some advantage over the epifaunal forms that lived at or close to the sediment-water interface. Nevertheless, the infaunal environment was not entirely unaffected since many of the taxa inferred to have been living there also disappeared. The abundance of benthic foraminifera (>150-µm fraction) was also severely reduced, although small individuals (<150 µm) remained abundant throughout. The ostracoda also exhibit a drastic decrease in diversity and abundance. The benthic foraminiferal assemblage was strongly depleted of coiled forms for several thousand years following the extinction. This left assemblages (>150 µm) temporarily dominated by relatively small, thin-walled, uniserial, triserial, and other forms more typical of an infaunal habitat (Corliss and Chen, 1988; Thomas, 1990). The relative increase in abundances of small benthic foraminiferal specimens, associated with a decrease in diversity, suggests conditions low in oxygen and higher in nutrients (Bernard, 1986; Thomas, 1990). Following a brief interval of extremely low diversity and abundance in the fraction greater than 150 µm (Figure 5.3), diversity increased again to about 30 species on average—a diversity of about half that before the extinction. This increase seems to have resulted mainly from the reappearance of forms that had been temporarily excluded from the benthic foraminiferal assemblage. Nevertheless,

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What can we expect as global change progresses? Will there be thresholds that trigger sudden shifts in environmental conditions—or that cause catastrophic destruction of life?

Effects of Past Global Change on Life explores what earth scientists are learning about the impact of large-scale environmental changes on ancient life—and how these findings may help us resolve today's environmental controversies.

Leading authorities discuss historical climate trends and what can be learned from the mass extinctions and other critical periods about the rise and fall of plant and animal species in response to global change. The volume develops a picture of how environmental change has closed some evolutionary doors while opening others—including profound effects on the early members of the human family.

An expert panel offers specific recommendations on expanding research and improving investigative tools—and targets historical periods and geological and biological patterns with the most promise of shedding light on future developments.

This readable and informative book will be of special interest to professionals in the earth sciences and the environmental community as well as concerned policymakers.

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