The event database

After a brief discussion of possible sources of error, this section summarizes how each event has been identified and quantified. Geological, paleoenvironmental, and paleobiological context is also given, as are relevant references. All data, including the identification of abbreviations, are provided in Table 1.

When multiple observations are used to compute a mean, the sample SD, the unaveraged observations, or both are noted in the event’s description. However, these quantities only indicate the variability of a particular set of observations rather than an estimate of the observational error. As with any compilation of isotopic data [for example, Hayes et al. (43)], inadequate sampling may be a more significant problem. Here, it takes two forms: the sheer paucity of observations, and biases deriving from not only the original investigators—who may favor larger isotopic excursions—but also the needs of the present compilation, which favors carbon isotopic studies associated with strong geochronological constraints. An additional source of error derives from incompleteness of the sedimentary record. For example, a hiatus in sediment deposition, or erosion after deposition, may make the true carbon isotopic minimum effectively unavailable to the modern observer, thereby leading to an underestimate of |Δ r |.

The ideal geochronological control would provide absolute dates at the initiation and termination of isotopic excursions. Only one event in the database—at the Ediacaran-Cambrian boundary—meets this standard. Among the others, all but 1 of the 18 events extending from the Toarcian (182 Ma) to the Miocene (16 Ma) are timed by astrochronology, including, in some cases, information obtained from isotope geochronology. The duration of five of the older, pre-Jurassic events is determined from at least two dates provided by U-Pb geochronology that bracket less than twice the accumulation of sediment deriving from the event itself; a sixth event (end-Permian) comes close to this standard.

The remaining pre-Quaternary events are timed by biostratigraphy with reference to the Geologic Time Scale (44); their time scales are therefore the most poorly constrained. Two of them, the end-Ordovician and Frasnian-Famennian events, are among the Big Five mass extinctions. A third mass extinction—the end-Triassic—is constrained by both isotope geochronology and astrochronology, but these constraints are not directly associated with the carbonate–carbon isotope excursion. If these three mass extinction events and the other four events with poor temporal constraints (Cambrian Spice, Tournaisian, mid-Capitanian, and Albian-Cenomanian) are removed from the database, the trends in Figs. 2 and 3 remain intact and no new trends appear.

Nevertheless, the unknown errors in the observations render quantitative error analysis infeasible. The representative error bars for Δ r , M, and τ env in Figs. 2 and 3 are instead guides for interpretation.

The following list proceeds from the oldest to the youngest event.

Ediacaran-Cambrian boundary, ~541 Ma. The magnitude and duration of this negative isotopic excursion derive from the Oman carbon isotope data of Amthor et al. (45) and the U-Pb geochronology of Bowring et al. (46). Other data from Morocco, Siberia, Mongolia, and China (47, 48) suggest global consistency. The absence of the Ediacaran biota in the Cambrian has led to the suggestion that the Ediacaran biota vanished by mass extinction at the end of the Ediacaran, possibly related to the environmental event signaled by this excursion (45, 49–52). However, gradual extinction coupled with permanent environmental changes unfavorable to preservation of the Ediacaran biota may also be possible (53).

Nemakit-Daldynian-Tommotian boundary, ~525 Ma. Both the carbon isotopic excursion and the U-Pb dates establishing the duration of this Early Cambrian event are from the Moroccan data of Maloof et al. (47, 54); data from Siberia, Mongolia, and China confirm its global nature (47, 48). No mass extinction has been associated with this interval. However, diversity (number of genera) and disparity (number of classes) sharply increase in the Tommotian, primarily because of the rise of small shelly fossils (55). The Nemakit-Daldynian-Tommotian boundary is also associated with a transition from seawater chemistry that favored aragonite precipitation to seawater favoring calcite precipitation (47, 56).

Cambrian Spice, ~497 Ma. The duration and magnitude of this late Cambrian event, also known as the Steptoean positive carbon isotope excursion, are taken from the Australian data of Saltzman et al. (57). The event has also been found in North America, China, Kazakhstan, and Sweden (58, 59). The initiation of this positive isotopic excursion coincides with an extinction of trilobites (58).

End-Ordovician event, ~446 Ma. The latest stage of the Ordovician—the Hirnantian—simultaneously exhibits a positive isotopic excursion and one of the Big Five mass extinctions. I estimated the magnitude of the negative limb of the excursion from data obtained at Vinni Creek and Monitor Range, Nevada (60), Copenhagen Canyon, Nevada (61), Blackstone Range, Yukon (60), Wangjiawan, south China (62), the Kardla drill core, Estonia (63), and Anticosti Island, Quebec (64). The seven data sets yield a mean excursion of 5.6 ± 0.9‰. Each data set except for Copenhagen Canyon has been correlated to graptolite zones by Gorjan et al. (62). By estimating the fraction of the persculptus and extraordinarius zones in which the excursion occurs at each site and estimating those time intervals from Gradstein et al. (44), I found that the negative limb of the excursion lasted approximately 230 ky.

Silurian Mulde event, ~428 Ma. A sequence of two positive excursions, known as the “Mulde event,” occurs in the Homerian stage of the Silurian, during a period of graptolite extinctions known as the “Big Crisis” (65). The event is apparently global (65). Cramer et al. (66) have recently bracketed the earlier of the two excursions between two U-Pb dates obtained in Gotland, Sweden. I focused on the negative downswing, which occupies somewhat more than half of the dated interval, and estimated its duration to be about 260 ky. As shown by Cramer et al. (66), the Gotland excursion correlates well with observations in the West Midlands, England, with the West Midlands excursion having a magnitude of about 4‰ and the Gotland excursion about 2‰, a range that is consistent with other observations (65, 66). I therefore estimated the magnitude to be 3 ± 1‰.

Silurian Lau event, ~423 Ma. A significant global positive carbon isotopic excursion known as the “Lau Event” occurs in the Ludlow Epoch of the Silurian (65, 67–72). The excursion typically peaks at 6 to 8‰ [for example, Lehnert et al. (71)], though values as small as 3 to 4‰ and as high as 10 to 11‰ have been observed in North America (68) and Sweden (70), respectively. Given that uncertainty, I adopted a conservative mean peak value of 7‰. The negative downswing after the peak typically bottoms out at about 1‰, resulting in a typical total negative shift of about 6‰. Cramer et al. (72) have recently bracketed the duration of the excursion by U-Pb geochronology, finding that it must be less than about 1.17 My. Following their analysis, I took the upswing and downswing to be roughly symmetric, each occupying about one-half of an approximately million-year interval, implying that the duration of the downswing is about 500 ky.

Frasnian-Famennian boundary, ~372 Ma. The boundary between the Frasnian and Famennian stages in the Late Devonian is associated with one of the Big Five mass extinctions (5, 6). But the drop in diversity at the boundary is thought to be a consequence of a lack of originations rather than an elevated extinction rate (19, 20). The boundary is associated with a global positive carbon isotopic excursion, known as the Upper Kellwasser Event (73). The magnitude and duration of the excursion are taken from the compilation of European Devonian data of Buggisch and Joachimski (74).

Tournaisian event, ~352 Ma. Buggisch et al. (75) presented carbon isotopic data from Belgium, Ireland, France, western Canada, and the western United States that collectively exhibit a pronounced positive isotopic excursion in the Tournaisian stage of the Carboniferous. Using their biostratigraphic time scale, I estimated the duration of the negative limb of the excursion to be 1.51 My; the magnitude of the excursion in their compilation is about 3.5‰.

Mid-Capitanian event, ~262 Ma. A mass extinction in the Capitanian Stage of the Middle Permian is associated with a negative isotopic excursion of carbonate carbon of about 5‰ over about 500 ky (76, 77). Observations of the event in south China suggest that the extinction event coincides with the initiation of Emeishan volcanism (78) and the onset of the excursion (76).

End-Permian extinction, ~252 Ma. The end-Permian extinction is considered the most severe of the Big Five (8). A significant negative isotopic excursion occurs at the time of the extinction, just below the Permian-Triassic boundary. Korte and Kozur (16) reviewed observations of the excursion in 40 localities worldwide and concluded that the excursion ranged from 4 to 7‰; I therefore took the magnitude of the excursion to be at the center of that distribution (5.5‰). The 60-ky duration of the excursion derives from the well-studied section at Meishan, south China, where Cao et al. (79) have provided high-resolution carbon isotopic data and U-Pb geochronology provides strong constraints on the time scale (9).

Early Triassic, ~251 Ma. Several significant isotopic excursions of unknown origin follow the end-Permian extinction (80–83). Galfetti et al. (83) provided carbon isotopic data tied to U-Pb geochronology from the Loulou Formation, northwest Guanxi, south China. The only temporally constrained negative isotopic shift occurs in a negative excursion that begins approximately 251.22 Ma (83) in the mid-Smithian and declines approximately 3.5‰ over approximately 250 ky.

Triassic-Jurassic boundary, ~201 Ma. One of the Big Five extinctions, the end-Triassic event is temporally associated with the emplacement of the Central Atlantic magmatic province (40). Geochemically, the most well-studied section is at St. Audries Bay in southwest England, where the extinction interval coincides with a rapid fall and subsequent rise of the isotopic composition of organic matter (84) that lasts 20 to 40 ky according to astrochronology (85). This so-called initial excursion in organic carbon is widely observed (86). However, its significance for understanding the evolution of marine dissolved inorganic carbon is unclear (86, 87), partly because it may be associated with observed changes in flora (87)—and therefore variations in the organic matter itself—and partly because inorganic isotopic compositions need not track organic isotopic compositions (43). There are unfortunately few well-resolved isotopic studies of carbonate carbon associated with the end-Triassic event (88–93). These carbonate studies also reveal a negative excursion associated with the extinction, but it is often unclear if they represent the “initial excursion” or the later 120-ky-long “main excursion” seen in organic carbon (84, 85). A recent review (94) cautions that diagenetic alteration may have corrupted carbonate data, yet a careful study of lithological effects at an Italian section suggests that the carbonate excursion may indeed be primary (87). An important additional problem is the need to estimate the time scale of the carbonate excursion. The data of Clémence et al. (92) provide a solution to these problems. In their analysis of inorganic and organic carbonate at the Tiefengraben section in the Austrian Alps, they found an initial 2‰ negative excursion in carbonate that extended to the second significant minimum in their organic data (92), which in turn correlates reasonably well with the astrochronologically calibrated organic data at St. Audries Bay (85). Their initial carbonate excursion was then found to last approximately 50 ky [which included one precessional cycle of the “pre-recovery” interval identified by Ruhl et al. (85)]. This 2.0‰ shift is also consistent with isotopic analyses of carbonate in pristine oysters located near St. Audries Bay (91).

Toarcian oceanic anoxic event, ~183 Ma. The early Toarcian oceanic anoxic event (95) is widely observed in Europe as a negative excursion in the isotopic composition of carbonate (96–99). To estimate its magnitude and duration, I used the high-resolution carbonate record of Hesselbo et al. (96) obtained at Peniche (Portugal). The astrochronology of Suan et al. (97) found a duration of 150 ky (their interval C1) for the negative isotopic excursion, which is consistent with the subsequent analysis of Huang and Hesselbo (100). Using this chronology, recent U-Pb dating by Burgess et al. (101) found that the intrusive magmatism associated with the Ferrar large igneous province is synchronous with the negative excursion.

Aptian oceanic anoxic event, ~120 Ma. The negative isotopic excursion associated with the Aptian oceanic anoxic event has been observed worldwide in open ocean records of carbonate carbon (102–105). My estimate of the magnitude and duration of the excursion derived from the astrochronology of Malinverno et al. (106) performed on the carbonate record of Erba et al. (103). The negative excursion spans the interval C3 [originally identified by Menegatti et al. (102)] and lasts approximately 47 ky.

Albian-Cenomanian boundary, ~100 Ma. This complex carbon isotope event occurred near the Albian-Cenomanian boundary (mid-Cretaceous) in several European sections (107). Here, I focused on the carbon isotopic data obtained at Ocean Drilling Program (ODP) Site 1050 at Blake Nose, western North Atlantic, as presented by Ando et al. (108). When plotted using the time scale of Huber et al. (109), the ODP data show an unambiguous 0.7‰ negative excursion across the boundary over approximately 110 ky.

Mid-Cenomanian event, ~95.9 Ma. The mid-Cenomanian positive carbon isotopic excursion has been clearly observed in European sections (107), the North Atlantic (110), western North America (111), and elsewhere. My analysis follows from the correlation of the “English Chalk” reference carbonate curve (107) to carbon isotopic data from the Cretaceous Western Interior Seaway of North America (111, 112). I used the time scale of Eldrett et al. (112), which is based on a combination of astrochronology and U-Pb geochronology. As indicated in figure 11 of Eldrett et al. (112), the negative swing of the excursion begins at about 96.5 Ma, followed by a decline of about 0.85‰ over roughly 138 ky.

Cenomanian-Turonian boundary event, ~94.2 Ma. Otherwise known as oceanic anoxic event 2, the positive isotopic excursion at the Cenomanian-Turonian boundary is probably the most widely observed of the Cretaceous isotopic events (107, 111, 112). As for the mid-Cenomanian event, my estimate of the time scale and magnitude of the Cenomanian-Turonian event follows from the correlation of the English Chalk reference carbonate curve (107) to carbon isotopic data from the Cretaceous Western Interior Seaway of North America (111, 112). I took the initiation of the event to be at the positive peak labeled B in figure 11 of Eldrett et al. (112) and found that the ensuing negative excursion declines by 1.60‰ in roughly 553 ky.

Cretaceous-Paleogene extinction, ~65.5 Ma. The end-Cretaceous negative isotopic excursion is associated with one of the Big Five extinctions (5), widely known for the extinction of dinosaurs and the Alvarez impact hypothesis (39). Its temporal association with the eruption of the massive Deccan volcanic province in India is less widely known (38). I obtained the magnitude and time scale of the carbon isotopic event from the deep-sea bulk carbon isotopic data obtained at ODP Sites 1210 (Northwest Pacific) and 1262 (South Atlantic), as presented by Alegret et al. (113) using an orbitally tuned time scale (114). I took the initiation and termination of the approximately 1.15 ± 0.03‰ (the mean of 1.16 and 1.13‰) negative isotopic excursion at each site to be where values of δ 1 begin to change by at least 0.1‰, resulting in a mean event duration of roughly 26 ky (the average of 8.5 and 44 ky).

Early late Paleocene event, ~58.9 Ma. Also known as the mid-Paleocene biotic event, this negative excursion is synchronous with dissolution of carbonate and changes in the organization of benthic and planktonic ecosystems (115, 116). The magnitude and time scale of this negative excursion are taken from the high-resolution bulk isotopic record of Littler et al. (115).

Paleocene-Eocene Thermal Maximum, ~55.5 Ma. The negative isotopic excursion of the Paleocene-Eocene Thermal Maximum is perhaps the most studied carbon isotopic event in Earth history (11), in large part because of its association with significant climate warming and clear evidence of ocean acidification (10). The event is also associated with a significant extinction of benthic foraminifera (12); however, other groups of benthic and planktic microfossils show little or no extinction (11). The time scales of the isotopic excursion of bulk carbonate in two deep-sea cores, from ODP Site 1266 on the Walvis Ridge in the South Atlantic and ODP Site 690 in the Weddell Sea, Southern Ocean, have each been estimated independently by identification of orbital cycles (117) and the estimation of sedimentation rates from the concentration of 3He (118, 119). The mean of the resulting four estimates [corresponding to the cumulative time between the onset of the excursion and the termination of its “core” as summarized in table 1 of Murphy et al. (119)] is 83 ± 23 ky. I obtained the magnitude of the isotopic excursion, 2.7 ± 1.1‰, from the mean of 33 published analyses of bulk Paleocene-Eocene Thermal Maximum carbonates reviewed by McInerney and Wing (11). The initial value of the excursion was obtained similarly (11). Although these averages lack prejudice, they may nevertheless underestimate the excursion’s size (120) and overestimate its time scale (121).

Eocene Thermal Maximum 2, ~53.7 Ma. The Eocene is punctuated by several “hyperthermal events,” each represented by a negative excursion in the isotopic composition of carbonate carbon and dissolution of deep-sea marine carbonates. Eocene Thermal Maximum 2, one such event, follows the Paleocene-Eocene Thermal Maximum (also known as Eocene Thermal Maximum 1) by roughly 2 My. My estimate of the magnitude of Eocene Thermal Maximum 2 derived from averaging estimates from four high-resolution bulk isotopic records presented by Stap et al. (122). The records derive from ODP Sites 1262, 1263, 1265, and 1267, corresponding to paleowater depths ranging from about 1500 to 3600 m (122); the shallowest site yields an excursion of about 1.5‰, whereas the others are each about 1.0‰ (122), yielding a mean of 1.13 ± 0.25‰. The time scale is about 45 ky.

Eocene Hyperthermal H2, ~53.6 Ma. The magnitude and time scale of this Eocene hyperthermal event come from ODP Sites 1263, 1265, and 1267. Following Stap et al. (122), I took the excursion of bulk carbon isotopes to be about 0.6‰ and the time scale to be about 33 ky.

Eocene Hyperthermal I1, ~53.2 Ma. I obtained the magnitude (0.72‰) and time scale (41 ky) of this Eocene hyperthermal event from the high-resolution bulk carbon isotopic record of Littler et al. (115), which derives from ODP Site 1262.

Eocene Hyperthermal I2, ~53.1 Ma. This Eocene hyperthermal follows Eocene Hyperthermal I1 by about 100 ky. Using the same source (115) as for Eocene Hyperthermal I1, I found an excursion of about 0.61‰ and a time scale of 40 ky.

Eocene Thermal Maximum 3, ~52.5 Ma. I estimated the magnitude and time scale of this hyperthermal event from the high-resolution benthic carbon isotopic record of Lauretano et al. (123), obtained at ODP Sites 1262 and 1263. The two records are similar, yielding an excursion of about 0.8‰ and a time scale of about 37 ky.

Early Oligocene Event, ~33.5 Ma. The positive carbon isotopic excursion just above the Eocene-Oligocene boundary is associated with the initiation of permanent Cenozoic ice sheets on Antarctica (124–126). I focused on the negative downswing to lighter isotopic compositions that followed. The time scale and isotopic change are derived from the high-resolution data collected at ODP Site 1218, using the astrochronological time scale presented by Coxall et al. (125). The negative shift extends over roughly 0.6 ± 0.1‰ during a period of about 430 ± 100 ky. The three ODP records presented by Zachos and Kump (126) are consistent with these estimates.

Miocene Climatic Optimum 1, ~16.9 Ma. Holbourn et al. (127) provided high-resolution carbon isotopic records spanning most of the Miocene Climatic Optimum, a period of warm climates ranging from 17.0 to 14.7 Ma that interrupted the longer-term trend of Cenozoic cooling. The records are obtained from Integrated ODP Site U1337 in the eastern equatorial Pacific Ocean, at a paleowater depth of 3500 to 4000 m. The Miocene Climatic Optimum began with a negative isotopic excursion, which I designated Miocene Climatic Optimum 1, that has also been identified in lower-resolution records from three other sites, elsewhere in the Pacific and in the Southern Ocean (127). I obtained the magnitude (0.5‰) and time scale (28 ky) from the bulk carbonate record. The event is synchronous with shoaling of the carbonate compensation depth (CCD).

Miocene Climatic Optimum 2, ~16.4 Ma. Holbourn et al. (127) identified three other Miocene Climatic Optimum negative excursions, which I designated Miocene Climatic Optimum 2 to Miocene Climatic Optimum 4, during which δ 1 decreases sharply and the CCD appears to shoal. Using the same high-resolution record as for Miocene Climatic Optimum 1, I found that Miocene Climatic Optimum 2 has a time scale of 22 ky and a magnitude of 0.76‰.

Miocene Climatic Optimum 3, ~16.0 Ma. See Miocene Climatic Optimum 2. For Miocene Climatic Optimum 3, I found a time scale of 32 ky and a magnitude of 0.66‰. This event also appears in the records of ODP Site 1146 in the western Pacific Ocean (128) and ODP Site U1338 in the eastern equatorial Pacific (129).

Miocene Climatic Optimum 4, ~15.6 Ma. See Miocene Climatic Optimum 2. For Miocene Climatic Optimum 4, the excursion has a magnitude of 0.6‰ and a time scale of 29 ky.