Osmium-isotope evidence for volcanism, weathering, and ocean mixing during the early Aptian OAE 1a Cinzia Bottini1,2, Anthony S. Cohen2, Elisabetta Erba1, Hugh C. Jenkyns3, and Angela L. Coe2 1
Dipartimento di Scienze della Terra “Ardito Desio”, Università degli Studi di Milano, Via Mangiagalli 34, 20133 Milan, Italy Department of Environment, Earth and Ecosystems, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK 3 Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK 2
1 GSA Data Repository item 2012176, Figures DR1–DR9, Tables DR1– DR5, and details of studied sections and methods, is available online at www .geosociety.org/pubs/ft2012.htm, or on request from
[email protected] or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.
mbsf
605
-6
60
0
( i)
(p
pb
)
s/ 1 88 O s
100 0.1
Re
7
O
18
Ca
δ 13
C
ca
CO
rb
3
(w
‰
t% )
(P
Lithology NC 7 Nannofossil zone Polarity chron
A
DB )
RESULTS Re and Os abundances of samples from both sites show similar trends (Fig. 1); abundances increase markedly during OAE 1a, reaching
0.7 0
20
610
Aptian Lower
625
D1 C
142
B
Nannoconid crisis Nannoconid decline
NC 6
D2
630 M0 -29
-19 0
-1
40
10
0
100 0.1
NC 7
2
0.7 0
S (wt%)
500 0
2
E D3
20
Selli Level
17
Aptian
D2
NC 6
C Nannoconid crisis
B M0 Lower
25
D1
A
35
Barremian
30
-30
-20 0
δ13Corg (‰) Lithology:
Black shale
10 TOC (wt%)
Marlstone
Limestone
2 0
0 192
Os (ppb)
Radiolarian bed
0.5
Nannoconid decline
645
NC 5
640
A
Barremian
DSDP Site 463
D3
620
635
B
Selli L. equivalent
E 615
NC 5
INTRODUCTION During the early Aptian, widespread accumulation of sediments rich in organic carbon occurred under oxygen-poor conditions throughout the world’s oceans. This phenomenon, known as Oceanic Anoxic Event 1a (OAE 1a), has its sedimentary expression in the Selli Level that crops out in the Apennines of Marche and Umbria, Italy (Coccioni et al., 1987), and in many other localities worldwide. It has been suggested that OAE 1a may have been triggered by the emplacement of the Ontong Java Plateau in the central Pacific Ocean (e.g., Erba, 1994; Larson and Erba, 1999; Jones and Jenkyns, 2001; Méhay et al., 2009; Tejada et al., 2009). The study by Tejada et al. (2009) found that the isotope composition of Os throughout one of the Selli sections in Italy (Gorgo a Cerbara) was exceptionally unradiogenic, suggesting that the Os was derived from a mantle, young basaltic, or meteoritic source. However, proof of a causal link between Ontong Java Plateau volcanism and the development of seawater anoxia has remained elusive because of the difficulty of correlating accurately the volcanic successions of the Ontong Java Plateau with the Tethyan sedimentary successions. Our study was therefore centered on the Re-Os isotope analyses of suites of samples from two of the key upper Barremian–lower Aptian sections across OAE 1a: Deep Sea Drilling Project (DSDP) Site 463 (Mid-Pacific Mountains, which is close to the Ontong Java Plateau volcanic center) and the distal Cismon core (southern Alps, northern Italy) (Fig. DR1 in the GSA Data Repository1). The sedimentary record of OAE 1a at Cismon is highly expanded relative to Gorgo a Cerbara; the two sections accumulated in different basins of the Tethys Ocean. The integrated stratigraphy and cyclochronology of the Cismon
core (Malinverno et al., 2010) provide an accurate time control, enabling us to correlate the two sites at high temporal resolution. Details of the sections and of the analytical methods are provided in the Data Repository.
CISMON
ABSTRACT The early Aptian Oceanic Anoxic Event (OAE 1a) resulted from an exceptional set of interactions between the geosphere, the biosphere, and the ocean-atmosphere system. We present new Re-Os data from two sites spanning OAE 1a in the Tethys and Pacific Oceans. The patterns of variation in the seawater Os-isotope composition from both sites are very similar, and together they constrain the timing and duration of continental weathering in relation to the large-scale volcanic activity of the Ontong Java Plateau. The dominant feature through the OAE is an interval of ~880 k.y. when the Os-isotope composition of the global ocean was exceptionally unradiogenic, implicating unambiguously the Ontong Java Plateau as the trigger and sustaining mechanism for OAE 1a. A relatively short-lived (~100 k.y.) Os-isotope excursion to radiogenic compositions in the Tethyan record is clearly linked to an abrupt perturbation to the global carbon cycle, and is fully consistent with the Pacific record. These highly distinctive features of seawater Os in contemporaneous samples from three high-resolution sections, two of which were very remote from the Ontong Java Plateau, indicate that ocean mixing at that time was very efficient. The results suggest that OAE 1a was also related to rapid global warming and elevated rates of silicate weathering both on the continents and in the oceans.
N (wt%) Ash layer
Chert
Figure 1. 187Os/ 188Os(i) (red diamonds), 192Os (pink circles), and Re (green squares), against stratigraphy (Erba et al., 1999, 2010; van Breugel et al., 2007; Ando et al., 2008; Malinverno et al., 2010). mbsf—meters below seafloor; PDB—Peedee belemnite; TOC—total organic carbon (shaded). Geochemical labels for both A and B are either at the top or bottom of the figure. A: Deep Sea Drilling Project (DSDP) Site 463. New δ13Ccarb (black) and δ13Corg (orange). B: Cismon core. S (violet diamonds) and N (blue circles).
GEOLOGY, July 2012; v. 40; no. 7; p. 583–586; doi:10.1130/G33140.1; 1 figure; Data Repository item 2012176. © 2012 Geological America. For permission to copy, contact Copyright Permissions, GSA, or
[email protected]. GEOLOGY 2012 | of www.gsapubs.org | July Society
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400 ppb for Re and 4.9 ppb for Os. No relationship is found between Re and 192Os abundances (the “common” Os component) with respect to either sulfur or total organic carbon (TOC) content, indicating that Re and Os are incorporated into Corg-rich sediments as a result of redox reactions (Figs. DR2 and DR3). Regression of the Re-Os isotope data for the Cismon samples (Fig. DR4) yields an age of 120 ± 3.4 Ma (MSWD = 190; 187Os/188Os(i) = 0.189 ± 0.019), which is very close to the age calculated for the Selli Level at Cismon (120.21 ± 0.04 Ma at the base; Malinverno et al., 2010). Samples from DSDP Site 463 yield a less precise age of 136 ± 19 Ma (MSWD = 1130; 187Os/188Os(i) = 0.170 ± 0.014) (Fig. DR5). Both ages are indistinguishable, within the ascribed analytical uncertainties, from the sedimentary ages for these sections, indicating that the Re-Os isotope system has remained closed since sediment deposition. The calculated 187Os/188Os(i) of samples is thus primary and reflects the Os-isotope composition of contemporaneous seawater (see Cohen et al., 1999). The samples from both sites studied here reveal patterns in seawater 187Os/188Os that are similar and coeval (Fig. 1), from which we define five segments, A to E. Segment A displays relatively constant 187Os/188Os (~0.5–0.6) except for a single unradiogenic value (0.35) in the lowermost part of the Cismon core. In segment B, a moderate decrease to less radiogenic ratios occurs before the onset of OAE 1a. In segment C, an abrupt and short-lived increase in 187Os/188Os to 0.52 occurred in the lowermost part of the Selli Level. At DSDP Site 463, a single unradiogenic value precedes the radiogenic Os excursion that reaches a maximum 187Os/188Os of 0.53. In segment D, there is a pronounced decrease in 187Os/188Os to an average value of ~0.16, commencing at the most prominent part of the negative carbon-isotope excursion (-ve CIE) and lasting through the entire Selli event with persistently unradiogenic 187Os/188Os. Segment D can be further divided into subintervals on the basis of minor fluctuations: D1 displays extremely unradiogenic 187 Os/188Os (~0.15) characterizing the -ve CIE interval; in D2, 187Os/188Os are slightly higher (~0.2) in the middle of the Selli Level; and in D3 there is a recurrence of extremely unradiogenic 187Os/188Os (~0.15) in the upper part of the Selli Level. The final segment E shows a relatively small increase in 187Os/188Os (~0.25) at the end of OAE 1a. DISCUSSION Re-Os Data from OAE 1a The Re-Os data from Cismon and DSDP Site 463 (Fig. 1; Fig. DR6) show little variation across segment A, suggesting that the dominant fluxes of Os (radiogenic Os from the continents and unradiogenic Os from the hydrothermal alteration of oceanic crust) were reasonably constant. One exception is in the lowermost part of the Cismon section, where a relatively unradiogenic value (187Os/188Os ≈ 0.35) occurs close to the level at which the nannoconid (heavily calcified nannofossil) abundance starts to decline. The paucity of samples analyzed in this interval allows us to speculate only very tentatively that an early pulse of Ontong Java Plateau volcanism, resulting in an input of unradiogenic Os, may have occurred at the start of a global biocalcification crisis (Erba et al., 2010). Additionally, Pb-isotope data from the Pacific Ocean support a latest Barremian age for the beginning of Ontong Java Plateau eruptions (Kuroda et al., 2011), at the same stratigraphic level as the nannoconid decline. Samples from segment A at Gorgo a Cerbara (Tejada et al., 2009) have higher 187Os/188Os(i) than samples of an equivalent age from both Cismon and DSDP Site 463; some of the Gorgo a Cerbara samples also contain unusually abundant Os. On a diagram of 187Os/188Os(i) against 1/ [Os] (Fig. DR7), the Gorgo a Cerbara segment A samples are located well away from the trend defined by the other samples from that location and by the samples from Site 463 and Cismon. Regression of the Re-Os data for the Gorgo a Cerbara segment A samples yields a highly imprecise age of 186 Ma, which is much older than their accepted geological age of
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ca. 120 Ma. These samples are from thin (~1 cm), fissile layers set within massive limestones and marls. The distinctive Re-Os characteristics of these samples, their lack of correct chronological information, and their field setting all suggest that their Re-Os signatures have been perturbed. The Os-isotope records from the two sites studied here (Fig. 1; Fig. DR6) confirm that the onset of OAE 1a was preceded by a decrease in the 187Os/188Os of seawater over a period of ~200 k.y. (segment B), as previously documented at Gorgo a Cerbara (Tejada et al., 2009). This observation implies that higher fluxes of unradiogenic Os commenced globally before the start of the OAE, and presumably reflects an early phase of Ontong Java Plateau volcanism. At Cismon, segment B is followed by a relatively short-lived (~100 k.y.) and pronounced radiogenic Os excursion (segment C) that correlates with the first part of the -ve CIE (Fig. 1; Fig. DR6). This shift in the 187Os/188Os(i), also observed at Gorgo a Cerbara (Tejada et al., 2009), could represent (1) a decrease in the unradiogenic Os flux caused by diminished Ontong Java Plateau volcanic activity, and/or (2) a substantial increase in the total flux of radiogenic Os supplied to the ocean through continental weathering. The documented rise in global temperature (e.g., Ando et al., 2008; Erba et al., 2010) during the first phase of OAE 1a is likely to have been responsible for accelerated weathering rates and the ensuing radiogenic Os-isotope excursion, with or without a temporary cessation in Ontong Java Plateau volcanic activity. Assuming as end members a value of 187Os/188Os = 0.127 (close to the presentday mantle composition) and a contribution from crustal weathering of 187 Os/188Os = 1.4 (Peucker-Ehrenbrink and Ravizza, 2000), to increase seawater 187Os/188Os to ~0.52 would require a 31% contribution of radiogenic Os from continental runoff, which implies an increase of ~72% in global weathering rates compared with pre-OAE conditions (segment B). Segment C is less well defined in samples from DSDP Site 463, perhaps due to incomplete core recovery and/or relatively low resolution of sampling, although the bio- and chemostratigraphic profiles indicate that there are no major hiatuses in this interval (Erba, 1994; van Breugel et al., 2007; Ando et al., 2008; Erba et al., 2010). Segment D represents a very pronounced and relatively long-lasting (~880 k.y.) anomaly in the Os-isotope composition of Early Cretaceous seawater (Fig. 1; Fig. DR6). Maintaining this exceptional seawater Os-isotope composition would have required a substantial shift in the balance of the Os fluxes to the oceans, most likely involving a large increase in the flux of unradiogenic Os. A cosmogenic origin can be ruled out since there is no evidence of a contemporaneous impact (Tejada et al., 2004). The input of unradiogenic Os is most likely to have resulted from the hydrothermal alteration of juvenile Ontong Java Plateau basalts, as suggested by Tejada et al. (2009). The other alternative— a substantial fall in radiogenic flux—is highly unlikely in view of the abrupt global warming and increased runoff at that time. The average seawater 187Os/188Os of ~0.16 in segment D requires an ~95% contribution of unradiogenic Os, implying an increase in the unradiogenic Os flux of ~25%–50% compared with pre-event values (segment A). This large unradiogenic Os flux is compatible with its supply through the high-temperature hydrothermal alteration of juvenile Ontong Java Plateau volcanics (average 187Os/188Os(i) ≈ 0.1295; Parkinson et al., 2001) and is in accord with the estimated age of basalts from this volcanic province (e.g., Tejada et al., 2007). The occurrence of persistently unradiogenic Os-isotope ratios throughout OAE 1a (segment D) in samples from both the Pacific and Tethys Oceans indicates that the global seawater Os-isotope composition was homogeneous, implying efficient oceanic circulation and mixing over this interval. The high-resolution stratigraphic calibration between the two sites demonstrates that the onset of segment D was coeval in the Pacific and Tethys Oceans (Figs. DR6 and DR8), thus implying that the main eruptive phase of the Ontong Java Plateau was of sufficient magnitude to dominate this aspect of seawater chemistry worldwide. Minor fluctuations
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within segment D (D1, D2, and D3) might have been due to small variations in volcanic intensity. There was an increase in seawater 187Os/188Os at the onset of segment E at Cismon, just before the end of the OAE (also recognizable at Gorgo a Cerbara; Tejada et al., 2009), suggesting that the intensity of Ontong Java Plateau volcanism decreased, perhaps accompanied by higher continental weathering rates. Integration with Biotic and Geochemical Proxy Data The new Os-isotope records can be linked with further information available for the two study sites (Fig. DR6). Segment B precedes the onset of OAE 1a and encompasses the nannoconid crisis, which is thought to have occurred as a consequence of increasing volcanogenic CO2 emissions from the Ontong Java Plateau (intervals 1–2 of Erba et al., 2010). The top of segment B corresponds to the beginning of the OAE and also to a major volcanogenic CO2 pulse (interval 3 of Erba et al., 2010). The most radiogenic Os-isotope composition in segment C immediately follows an interval of abrupt warming and the inferred injection of CH4 into the ocean-atmosphere system (interval IV of Méhay et al., 2009), which may have been instrumental in triggering this phase of intense continental weathering. Indeed, the middle and upper part of segment C correlates with a cooling interlude and nannofossil carbonate recovery (interval 7 of Erba et al., 2010), interpreted as a consequence of lowered CO2 due to accelerated weathering. At Resolution Guyot (Mid-Pacific Mountains), a single relatively radiogenic Sr-isotope data point has been recorded at the level of the -ve CIE (Jenkyns et al., 1995), which correlates with segment C in the Tethyan sections. Ca-isotope data from the same site similarly indicate, across the equivalent time interval, an increase in continental weathering rates (Blättler et al., 2011). DSDP Site 463 is located close to Resolution Guyot, so the Pacific Os-isotope record of segment C might indeed contain a record of the intensified continental weathering that is displayed so clearly in the Tethyan sections. In this regard, we note that the similarities displayed by the Tethyan and Pacific Os-isotope records contrast markedly with Pb-isotope data (Kuroda et al., 2011), which demonstrate the lack of an Ontong Java Plateau Pb-isotope signature in Tethyan records. The onset of the maximum phase of carbonate dissolution and warming (interval 8 of Erba et al., 2010) corresponds to the rapid decrease in seawater 187Os/188Os that culminates in the exceptionally unradiogenic Os-isotope segment D. This interval most likely represents the onset of the most intense phase of Ontong Java Plateau volcanism, and occurs at precisely the same point as does evidence for progressive surface-water acidification, biocalcification failure within calcareous nannoplankton, and progressive shoaling of the calcite compensation depth (CCD) (Erba et al., 2010). The onset of a relative decrease in magmatic activity in relation to weathering processes (D2) coincides with recovery of the nannoplankton population and the likely deepening of the CCD (interval 12 of Erba et al., 2010). We suggest that a temporary decrease in volcanogenic CO2 emissions favored a partial nannofossil recovery, possibly also promoted by CO2 drawdown through increased continental weathering. The intervals of increased Ontong Java Plateau volcanism are also consistent with marked variations in trace-metal abundance and a sharp decrease to relatively unradiogenic Sr-isotope ratios, both of which suggest substantially increased submarine hydrothermal activity (e.g., Larson and Erba, 1999; Jones and Jenkyns, 2001; Duncan et al., 2007). Comparison with Other OAEs and Hyperthermals Comparison of the seawater Os-isotope record of OAE 1a with data for other Mesozoic OAEs shows some striking similarities and also important differences (Fig. DR9). For OAE 1a, there is evidence that the onset of anoxia and the negative δ13C anomaly were preceded by a significant input of unradiogenic Os (Tejada et al., 2009; this study). Similarly, unradiogenic 187Os/188Os (Turgeon and Creaser, 2008) predate the latest
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Cenomanian OAE 2 (94–93.6 Ma; Ogg et al., 2008) and the positive δ13C anomaly. These observations are fully consistent with volcanism acting as a trigger for global anoxia. The Os-isotope data indicate major eruptive phases lasting for almost the entire duration of these events, namely ~880 k.y. for OAE 1a (this study) and ~550 k.y. for OAE 2 (Turgeon and Creaser, 2008), thereby maintaining anoxic conditions over unusually large areas of the oceans for a prolonged period. However, seawater 187 Os/188Os indicate that during OAE 1a, Ontong Java Plateau volcanism was intense right up to the end of OAE 1a; conversely, the published data suggest a progressive decline in volcanic activity during OAE 2 after the first ~190 k.y. (Turgeon and Creaser, 2008). Accelerated continental weathering and increased runoff, implicated as the cause of radiogenic Os-isotope excursions, have been documented not only for OAE 1a (Tejada et al., 2009; this study) but also for the Toarcian OAE (T-OAE, ca. 183 Ma) (Cohen et al., 2004, 2007) and the Paleocene–Eocene Thermal Maximum (PETM, ca. 55.5 Ma) (Ravizza et al., 2001). In all three cases, the radiogenic Os-isotope excursions correspond to abrupt global negative shifts in δ13C that were very likely to have been a consequence of CH4 and/or CO2 release into the ocean-atmosphere system, and consequent global warming. For OAE 2, no radiogenic Os-isotope spike has yet been detected; however, Sr- and Ca-isotope data (Frijia and Parente, 2008; Blättler et al., 2011) provide evidence for an increase in global weathering at the onset of OAE 2. The duration of the radiogenic Os-isotope interval is ~100 k.y. for both OAE 1a and the PETM (Zachos et al., 2005), while the T-OAE is likely to have lasted between 168 and 324 k.y., depending on interpretation of the cyclostratigraphy (Kemp et al., 2011). It is noteworthy that the large-scale volcanic events that were associated with the T-OAE (the Karoo-Ferrar igneous province) and the PETM (the North Atlantic igneous province) were predominantly continental and subaerial. The introduction of metals and nutrients to the oceans would thus have required the prior weathering of large volumes of subaerially erupted basalt (e.g., Cohen et al., 2004). In contrast, the emplacement of the Ontong Java Plateau was predominantly submarine (Tarduno et al., 1991; Kuroda et al., 2011), and thus the direct introduction of many metals, including unradiogenic Os and nutrients, would have been effectively instantaneous. Despite the differences in volcanic style and the very distinct background conditions of climate and paleogeography, many of the major responses of the Earth system during OAEs and the PETM were remarkably similar. CONCLUSIONS The new Os-isotope data demonstrate unequivocally that the effects of submarine Ontong Java Plateau volcanism were global and that volcanism was directly implicated in the development of OAE 1a. These observations also imply efficient oceanic circulation and mixing during OAE 1a. Correlation of the chemostratigraphic data of this study with published cyclostratigraphy for OAE 1a enables us to constrain the timing and duration of the major phases of Ontong Java Plateau emplacement and to document its interaction with both the marine and terrestrial realms. These observations demonstrate that high-resolution records of past global warming can provide valuable information about the behavior of the Earth system during and after the large-scale release of carbon into the ocean-atmosphere system. ACKNOWLEDGMENTS We thank Marc Davies for his unstinting and expert help with all aspects of sample preparation and analyses, John Watson for LECO elemental analyses (The Open University, UK), and Normal Charnley and Peter Ditchfield for undertaking C-isotope analyses (Oxford). We also thank D. Selby and an anonymous reviewer for their helpful comments on the manuscript. Bottini and Erba were funded through grant MIUR-PRIN-2007-2007W9B2WE 001. Bottini was supported by a Cariplo Foundation grant. Analytical work at The Open University was supported by funds from NERC and The Open University. Samples from
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Manuscript received 20 December 2011 Manuscript accepted 27 January 2012 Printed in USA
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GEOLOGY
