The Pliocene was a time when global climatic conditions were in full decline, changing from the Greenhouse Earth of the early Tertiary to the Icehouse Earth of the late Neogene and Quaternary. Climatic change has a direct effect on the temperature of oceanic surface waters. Many different proxies have been used to assess the changing temperature of oceanic waters, and thus climate, with time (e.g., oxygen isotope ratios, alkenone measurements, temperature-sensitive single species or assemblages of various microfossils, structural changes in microfossil skeletons, and so on). In this study, we have used a technique employing two temperature-diagnostic nannofossil species in an attempt to identify trends in surface-water temperature changes in the western Mediterranean during the Pliocene.
Calcareous nannoplankton are organisms that live in the upper surface waters of the oceans and are thus directly influenced by surface-water changes. The discoaster group has long been known to prefer warm waters, and several earlier workers have produced paleotemperature studies using the ratio of warm-water discoasters, as a group, to cool-water Chiasmolithus or Coccolithus (e.g., Bukry, 1978, 1981; Haq et al., 1977; Siesser, 1980, 1984; Raffi and Rio, 1981). In the Neogene, however, several discoasters (D. variabilis, D. intercalaris, D. tamalis, and D. asymmetricus) are believed to have preferred cool waters (Bukry, 1981; Rio et al., 1990a). A single discoaster species, Discoaster brouweri, which has a well-established preference for warm waters (e.g., Bukry, 1978, 1981, Siesser, 1975; Muller, 1985, Wei et al., 1988), was thus selected as the warm-water proxy for this study.
Coccolithus pelagicus was chosen as the cool-water proxy. Coccolithus pelagicus lives only in cold-temperate (6°-14°C) northern hemisphere waters today (Mcintyre et al., 1970; Raffi and Rio, 1981), but has apparently changed its habitat with time. In Miocene and earlier Tertiary sediments, C. pelagicus was common in tropical environments as well as in cooler waters (Bukry, 1981). Bukry (1981) made a careful analysis of the distribution of this species, concluding that by the Pliocene C. pelagicus had evolved an affinity for cool water that made it an effective proxy for determining paleotemperature trends. Raffi and Rio (1981) also concluded that C. pelagicus was a good paleotemperature indicator for the Mediterranean during the late Pliocene.
Discoaster brouweri and C. pelagicus have additional advantages in that they are both normally significant components of nannofossil assemblages, and they are also less affected by diagenetic changes than many other species. The changing downhole ratio of these two nannofossil species should, therefore, be a good paleotemperature indicator, reflecting gross changes in water temperature with time. Using a ratio rather than absolute abundances of the two species avoids potential error caused by variations in the density of specimens sedimented on a slide. Wei et al. (1988) also used the ratio of these two species to investigate Neogene water temperatures on the Galicia Margin.
In this study, we counted 100 specimens of D. brouweri and C. pelagicus along random traverses across slides from each Leg 161 Pliocene core. We included all varieties of "D. brouweri" in our count of D. brouweri s.l.: D. brouweri var. rutellus Gartner; D. brouweri subsp. bipartitus Haq and Berggren; D. brouweri subsp. brouweri Theodoridis; D. brouweri subsp. streptus Theodoridus, and D. triradiatus Tan. Species counts for each sample examined are shown in the Appendix. Results are expressed as the percentage of D. brouweri in the total count of 100 D. brouweri and C. pelagicus specimens, and are plotted against nannofossil zones in Figure 9, Figure 10, and Figure 11.
The D. brouweri-C. pelagicus ratio is generally low in the early part of Zone NN12, suggesting cool waters (Fig. 9). In early Zone NN13 waters warmed considerably, before becoming cool again in late NN13. Temperatures began to rise in Zone NN14 and continued to be relatively high throughout the NN14-NN17 interval. Zones NN16A, NN16B, and NN17 show a major warming trend, with a sample in NN16B (974B-13H-3, 27-28) showing the highest ratio measured in any Leg 161 hole (D. brouweri = 47; C. pelagicus = 53). Zone NN18 shows decidedly cool waters. Raffi and Rio (1981) used the abundance of C. pelagicus and discoasters (as a group) to estimate relative temperatures for the Pliocene at DSDP Site 132. The data presented in our study only partially agree with the data presented by Raffi and Rio (1981). This may be because different proxies were used, as well as zonations differing in their degree of resolution, making close comparison difficult. The marked increase in discoasters/D. brouweri during the late Pliocene followed by the precipitous reduction of these forms in Zone NN18 is, however, common to both studies.
We believe the sharp warm peak centered on Zone NN16B is the mid-Pliocene warm interval described in several recent papers (e.g., Crowley, 1996; Dowsett et al., 1996).
The D. brouweri values in this hole fluctuate erratically, and lack clear trends (see Appendix; Fig. 4). The largest warm-water peaks are in early Zone NN13, late NN15-early NN16A, and NN17.
Zones NN13 to NN17 are missing in this section. Ratios in the remaining intervals show a marked warm-water peak in latest Zone NN12, with cool waters in Zone NN18 (see Appendix; Fig. 5).
A brief cool-water interval in early Zone NN12 was followed by generally warm waters from the middle of NN12 into early Zone NN13 (Fig. 10). Waters were cool in late Zones NN13 and NN14, with warming occurring briefly again in Zone NN15, followed by a brief cooling in late NN15-early NN16A. A period of sustained warming followed which peaked in Zone NN16B. The brief, but markedly warm period in earliest NN16B is the mid Pliocene warm interval (Crowley, 1996; Dowsett et al., 1996). Waters cooled in Zones NN17 and NN18.
This hole contains the most expanded stratigraphic section obtained during Leg 161 (Fig. 11). Cool waters in Zone NN13 were followed by fluctuating cool and warm intervals in NN14 and NN15. Sustained warming began in late Zone NN16A and continued into NN16B. The warmest waters of the Pliocene occurred in the middle Pliocene (late NN16A-NN16B). Waters began to cool significantly in late NN16B, a trend that continued into Zones NN17 and NN18.
This section ranges only from Zones NN16A to NN18. Warm waters in early Zone NN16A became cooler in late Zone NN16A. Alternately warm- and cool-water peaks fluctuate erratically and closely in time in Zone NN16B, although this zone is overall the warmest interval of the Pliocene at this site (Appendix; Fig. 8). Zones NN17 and NN18 were cool-water intervals.