Did Cooling Oceans Trigger Ordovician Biodiversification? Evidence from Conodont Thermometry

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Science  25 Jul 2008:
Vol. 321, Issue 5888, pp. 550-554
DOI: 10.1126/science.1155814


The Ordovician Period, long considered a supergreenhouse state, saw one of the greatest radiations of life in Earth's history. Previous temperature estimates of up to ∼70°C have spawned controversial speculation that the oxygen isotopic composition of seawater must have evolved over geological time. We present a very different global climate record determined by ion microprobe oxygen isotope analyses of Early Ordovician–Silurian conodonts. This record shows a steady cooling trend through the Early Ordovician reaching modern equatorial temperatures that were sustained throughout the Middle and Late Ordovician. This favorable climate regime implies not only that the oxygen isotopic composition of Ordovician seawater was similar to that of today, but also that climate played an overarching role in promoting the unprecedented increases in biodiversity that characterized this period.

Oxygen isotopes provide a valuable means for tracking environmental change, with oxygen isotopic compositions of marine fossil carbonates (δ18Ocarb) being especially useful for reconstructing Cenozoic sea-surface temperatures (1). This approach, however, has proven less reliable for older biogenic carbonates, which are more prone to diagenetic modification. Anomalously low oxygen isotope compositions of Early Paleozoic biogenic carbonates have driven contentious debate about the implied high seawater temperatures and variability in seawater oxygen isotopic composition (δ18Oseawater). Resolution of these issues has important implications for understanding fundamental Earth processes and major events in Earth history.

The Paleozoic marine δ18Ocarb record is derived mainly from calcitic brachiopods (2, 3) that exhibit a wide range in composition from about –2 per mil (‰) to –10‰ Vienna Pee Dee belemnite (V-PDB). It has been argued that this record reflects secular variations in global climate (2, 4) but, assuming present-day values for seawater (δ18Oseawater ∼ –1), the calculated temperatures are unrealistically high, reaching 70°C in the Early Ordovician. This seeded further speculation that δ18Oseawater composition, and by implication hydrothermal processes, evolved with time (3, 57). This is due to the inherent limitation of the oxygen isotope proxy being a combined signature of temperature and seawater isotopic compositions, thereby comprising two unknowns. The proposition that δ18Oseawater changed significantly over time is, however, inconsistent with altered seafloor basalt compositions and models of the seawater isotopic budget, which imply that δ18Oseawater has remained essentially constant since the Archean (811). This suggests that these older biogenic carbonates do not reflect primary paleoseawater compositions, yet these issues remain contentious.

The temperature record of Ordovician oceans is central to understanding links between seawater chemistry, climate change, major bio-events, and thus fundamental Earth processes. The marine biosphere underwent a profound transformation during the Great Ordovician Biodiversification Event (GOBE), recognized as the longest period of sustained biodiversifications, increasing family and genus numbers three- to fourfold (12) (Fig. 1). This unparalleled event is characterized by the replacement of the Cambrian Evolutionary Fauna with considerably more complex Paleozoic and Modern Evolutionary Faunas. The GOBE was terminated with sudden and catastrophic extinctions during the latest Ordovician (Hirnantian), probably associated with rapid ice sheet growth at the South Paleopole (13).

Fig. 1.

Biodiversity patterns of marine fauna through geological time. Middle Cambrian to Silurian (except Pridoli) taxonomic diversity trends at genus level [modified from Sepkoski (15)]. Inset shows Phanerozoic taxonomic diversity of marine faunas at family level [modified from Sepkoski (12)]. Cambrian: M, Middle; U, Upper. Ordovician T, Tremadoc; Ar, Arenig; Ln, Llanvirn; C, Caradoc; As, Ashgill. Silurian: Lly, Llandovery; W, Wenlock; Lw, Ludlow.

Although there are different regional biodiversification patterns, which are well recognized from biotal provincialism, at a higher level there are important global trends. Major increases in biocomplexity and biodiversity began in the Early Ordovician with notable expansions throughout the pelagic realm. The onset of the first major biodiversity surge occurred during the Mid Ordovician with extensive colonization of the benthos including the establishment of hardgrounds and reefal systems (14), although diversifications of many groups peaked throughout the Late Ordovician (12, 14, 15). The causal mechanism(s) that drove these radiations has been elusive and enigmatic given the long-standing belief that supergreenhouse conditions prevailed. Clearly, local and regional environmental conditions (e.g., sedimentation, eustasy, temperatures, nutrients, ocean circulation) would have substantially shaped the character of evolving marine communities, as would various biophysical mechanisms. However, overarching global conditions (e.g., climate, sea level) would have also played key roles in this major reconfiguration of the marine biosphere.

To better characterize the Ordovician climate regime, we have used conodont apatite as a potentially robust temperature archive. Although not as abundant or easy to analyze as carbonate, the phosphate mineralogy of conodont microfossils is more stable than that of biogenic marine carbonates. Furthermore, conodonts are ubiquitous in Cambrian-Triassic marine sequences world-wide and evolved rapidly, providing fine stratigraphic resolution. A major drawback, however, is that conodonts are small (∼0.1 to 3 mm long), so previous analyses (16, 17) have typically required “bulk” sampling, even those using infrared laser isotope ratio monitoring gas chromatographic mass spectrometry (18). This is less than ideal because such samples may contain contaminants and remnant basal tissue that can compromise the analysis (18). Moreover, compositional heterogeneity between different taxa will not be discernible by bulk analyses, thereby requiring analysis of monospecific samples that are rarely available in sufficient volume. Bulk analyses of Silurian-Carboniferous conodonts have nonetheless yielded oxygen isotope compositions [δ18Ophos = ∼18 to 23‰ Vienna standard mean ocean water (V-SMOW)] (19) giving plausible paleotemperatures (∼33° to 17°C) (1618, 20, 21), encouraging further exploration of the technique.

Here we show the feasibility of in situ oxygen isotope analysis of single conodont elements at a 30-μm scale using the SHRIMP II ion microprobe at The Australian National University (ANU), recently configured for high-precision stable-isotope analysis (22). Durango apatite was used as the primary isotope standard, its composition independently determined by gas isotope ratio mass spectrometry (δ18Oapatite = 9.4‰). The tooth enameloid of a modern great white shark (δ18Oapatite = 22.3‰) was used as a secondary standard. The standard deviation of replicate analyses of these standards ranged from 0.42 to 0.14‰ within sessions, with higher precision reflecting ongoing technical improvements. The conodonts (179 analyses, 102 specimens) were sampled from 20 Ordovician to Early Silurian temporal horizons from 8 sites of similar tropical paleolatitudes (23) across Gondwana (Australia) and Laurentia (Canada), of primarily shallow subtidal (with two distal slope) facies (22). Consistent shallow-water sampling avoided isotopic variability that would likely be expressed by mixed biofacies. Mean population compositions (Fig. 2) were determined with a precision of typically 0.5‰ (95% confidence level). At this precision, no pronounced isotopic differences were found between tissue types (hyaline or albid) within individual conodont elements, or between most elements within a single sample population. A larger range of compositions was found in some Hirnantian conodont populations, and in an older sample (Manitoba) possibly affected by hypersaline conditions.

Fig. 2.

Oxygen isotope compositions of Ordovician to Early Silurian conodonts and brachiopods. Ordovician and Silurian conodont bioapatite compositions measured during this study using SHRIMP II are compared with compositions previously reported from conodonts (18, 24) and calcitic brachiopods (3, 6). Conodonts analyzed using SHRIMP II are from 20 stratigraphic horizons at 8 different sites across Canada and Australia. Ordovician time scale from Webby et al. (14) with new stage names from the International Subcommission on Ordovician Stratigraphy; Dap, Dapingian; H, Hirnantian; Rhud, Rhuddanian; Aeron, Aeronian; Telych, Telychian. Error bars for Ordovician brachiopod data (3, 6) are ±1σ (gray squares and crosses), all other data are shown at ±2σ; errors for some means were unavailable; Bassett et al. 2007 (24) data normalized to NBS 120c = 21.7‰ V-SMOW to allow comparison with our data. Gray band represents the primary first-order temporal trend of Ordovician δ18Oapatite determined by this study.

The measured conodont δ18Oapatite compositions are internally consistent across geographically disparate sites from two cratons, with no systematic patterns related to facies, and are thereby interpreted as a global temporal trend (Fig. 2). Furthermore, the earliest Ordovician conodonts from Australia have compositions equivalent to those from Texas reported by Bassett et al. (24), and the Wenlock data from Cornwallis Island are consistent with coeval samples from Gotland (18). Throughout the Ordovician, δ18Oapatite increased from ∼15.3 to ∼19.6‰ (V-SMOW), equivalent to temperatures from ∼42° to ∼23°C (δ18Oseawater = –1). Our δ18Oapatite record provides a first-order primary trend of Ordovician climate variability that discriminates four main climate regimes: (i) sustained cooling during the Early Ordovician (∼15.3 to ∼18.3‰), reflecting a global shift from greenhouse conditions (∼42°C) to modern equatorial temperatures (∼28°C) over ∼25 million years (Myr); (ii) climate stability with modern equatorial temperatures for ∼20 Myr from the Mid through Late Ordovician; (iii) a rapid temperature drop (25) during the Hirnantian, marking the well-known latest Ordovician glaciation; and (iv) a return to modern equatorial temperatures by the early Wenlock (Fig. 3). Higher– temporal resolution studies will be required to characterize finer-scale climate variability throughout this period.

Fig. 3.

Generalized global biodiversity pulses and tropical seawater temperature trend through the Ordovician. Temperatures derived from conodont oxygen isotope compositions measured in situ using SHRIMP II show a unidirectional cooling phase and a period of sustained moderate temperatures. Temperature means are plotted for repeat analyses of Canning Basin and Manitoba samples that were determined during different analytical sessions. Blue trend-line represents the primary first-order temporal trend of Ordovician sea-surface temperatures estimated from this study. Yellow band highlights first moderate temperatures and accompanying major biodiversity pulses. Previously reported conodont data (18, 24) are shown together with Silurian carbonate clumped-isotope data (27). Timing of biodiversity pulses shown by thickened vertical lines [compiled from data in Webby et al. (14)], line thickness not proportional between groups, diversity peaks are not represented and vary regionally. Some trilobites are pelagic; I denotes Ibex Fauna, W denotes Whiterock Fauna; algae comprises calcified reds, greens, and cyclocrinids; graptolites based on Chen et al. (31) and include new planktonic forms, the Graptoloidea; miospores represent terrestrial flora; temperatures were calculated assuming δ18Oseawater = –1‰ V-SMOW, with no adjustment for ice-volume effects for Hirnantian Anticosti Island data, and temperatures of Bassett et al. 2007 (24) based on per mil values normalized to NBS 120c = 21.7‰ V-SMOW.

This conodont δ18Oapatite record (Fig. 2) indicates that sea-surface temperatures for much of the Ordovician to Early Silurian were well within present-day ranges (Fig. 3). Even our highest temperature estimate (∼42°C), when considering analytical (∼3°C range) as well as δ18Oseawater uncertainties, is essentially at the upper limit of present-day surface waters (e.g., Red Sea, Persian Gulf, Sunda Sea). This is consistent with suggestions that the Late Cambrian to Early Ordovician was a higher-temperature interval that favored microbial buildups and exceeded the tolerance of many stenothermal reef-associated organisms (26). The Ordovician δ18Oapatite record, however, contrasts markedly with earlier perceptions of a predominantly “supergreenhouse” state, and the carbonate record (2, 3) of considerably lower and dispersed δ18Ocarb values, which implies that those samples were dominated by secondary alteration processes. This negates arguments that hydrothermal alteration processes controlling δ18Oseawater have changed appreciably over time. Furthermore, our conodont climate record is inconsistent with the Early Paleozoic “icehouse” and “greenhouse” modes inferred from the detrended δ18Ocarb record (4), which are exaggerated and temporally expanded. Collectively, the restricted range in δ18Oseawater (±2‰) evidenced by seafloor basalt compositions, and modern-like values (δ18Oseawater = –1) suggested by conodont apatite and carbonate clumped-isotope thermometry (27), show that there is no physical basis for linearly detrending the δ18Ocarb record, thereby invalidating models founded on that approach (4, 28).

The Early to Mid Ordovician cooling phase reflects an important change in Early Paleozoic climate, which likely played a key role in determining marine biodiversity patterns (Fig. 3). Perhaps most notable was the widespread biotic colonization of the planktonic realm from a limited benthic mode, as illustrated by the evolution of Cambrian dendroid graptolites to Ordovician planktonic anisograptids (29), with a similar pattern expressed by radiolarians (30). It has also been proposed that planktotrophy may have evolved during the Early Ordovician in response to pressures of benthic predation (31). The clear paleontological evidence of increased biomass throughout the water column and consequent increased carbon burial, an important controller of atmospheric CO2 concentration, probably contributed to this cooling trend.

The cessation of cooling and stabilization at modern equatorial temperatures indicates that a new equilibrium between partial pressure of CO2 (pCO2) and biological productivity had been reached by the Mid Ordovician. This new moderate climate regime likely spurred major biodiversifications by providing more favorable conditions consistent with modern-day equatorial temperatures (Fig. 3). Expansions were especially prolific in the benthic realm, including new complex metazoan-algal reefal communities and the circumglobal establishment of coral reefs by the mid Late Ordovician (14). Such widespread carbonate biomineralization possibly reflected increased seawater carbonate saturation due to decreasing pCO2 (32), and a cooler moderate climate regime consistent with the thermal tolerance window of modern corals. This plausible scenario contrasts with recent suggestions that asteroid impacts triggered the Mid to Late Ordovician biodiversifications (33), which offer no credible mechanisms and are typically associated with extinction events.

The Late Ordovician temperature fall in the δ18Oapatite record coincides with the Hirnantian glaciation (time slice 6c; Fig. 3), but suggests a considerably smaller shift on the order of ∼1‰ (∼4°C), compared to the ∼4‰ increase determined from biogenic carbonates from Baltica (34). Either our sampling through this event is incomplete, or the Baltic record is affected by local aberrations and/or diagenesis (35). The actual temperature range, however, is difficult to ascertain given that the ice-volume component, which was clearly substantial during this interval, cannot be discriminated (36). Notably, the Hirnantian conodont populations tend to have larger ranges in isotopic compositions between specimens (22), as confirmed by replicate analyses. Such variations may be attributable to any combination of factors including “vital effects,” seasonal variations, and analytical error. Further analyses at high temporal resolution are required to better constrain the duration and magnitude of this event, although the current data are inconsistent with a protracted cooling event (37), and clumped isotope thermometry could help constrain ice-volume effects. The dramatic extinctions (the second-largest in Phanerozoic history) that resulted from this rapid glaciation might partly reflect the lack of adaptability of Ordovician biota not previously subjected to such low temperatures.

Although few, the Early Silurian data are consistent with the Mid to Late Ordovician ranges (Figs. 2 and 3), with the transient low Rhuddanian temperature (∼24°C) implying climate instability before returning to modern equatorial conditions in the Wenlock (∼30°C). Conodont-derived Silurian temperatures are significantly lower than those based on coeval brachiopods (6, 18), again indicating that the oxygen isotope compositions of the carbonates have been compromised (Fig. 2). Recent clumped-isotope thermometry of slightly older calcitic brachiopods (27) suggest somewhat warmer conditions (∼35°C) for the Telychian (Fig. 3). The latter approach has the advantage of determining temperatures independent of seawater isotopic composition, as well as estimating the δ18Oseawater, but the robustness of this technique has yet to be fully validated.

Our δ18Oapatite record demonstrates both the robustness and benefits of high–spatial resolution in situ oxygen isotope analysis of discrete conodont elements, with continued technical advances promising even higher precision and accuracy. Our new global climate record suggests that Early Ordovician “greenhouse” temperatures cooled to present-day conditions that characterized the remainder of the period, suggesting that δ18Oseawater has remained essentially invariant. Furthermore, climate amelioration is coincident with widespread taxonomic radiations, which marked one of the most important evolutionary developments in Earth history. Although temporally and spatially varied, these expansions in biomass and biodiversity were not only modulated by local environmental controls and inherent biophysical mechanisms, but were likely initiated by this favorable climate regime.

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