The early Aptian Oceanic Anoxic Event 1a (OAE 1a: ~120 Ma) is a global phenomenon of organic matter burial in oxygen-depleted oceans. Volcanism, associated with the emplacement of the Ontong Java Plateau (OJP), is thought to be the main triggering mechanism for global anoxia, ocean acidification and greenhouse conditions (e.g. ERBA, 1994; LARSON & ERBA, 1999; JONES & JENKYNS, 2001; MÉHAY et alii, 2009; TEJADA et alii, 2009; BOTTINI et alii., in press). Independent studies on TEX86, sporomorphs and oxygen isotopes (e.g. MENEGATTI et alii., 1998; DUMITRESCU et alii., 2006; ANDO et alii, 2008; KUHNT et alii., 2011; MILLAN et alii., 2011; KELLER et alii., 2011) provided evidence for climate instability during OAE 1a. Nevertheless, the direct/indirect role of OJP volcanism, weathering rates and pCO2 on climate changes has not been established. We have performed a high-resolution integrated nannofossil-geochemical investigation of OAE 1a in three sites: Cismon core (Italian Southern Alps), Piobbico core (Umbria-Marche basin, Italy) and DSDP Site 463 Mid-Pacific Mountains. The existing stratigraphic framework for the three sites and available cyclochronology for the Cismon core (MALINVERNO et alii., 2010) allows high-resolution dating of biotic and environmental fluctuations. The nannofossil Temperature Index correlated with the oxygen-isotope record has revealed systematic and synchronous changes in the three sites. A warming pulse is registered prior to the onset of OAE 1a and corresponds to a prominent volcanic phase documented in the Os-isotope records (TEJADA et alii., 2009; BOTTINI et alii., in press). During OAE 1a, in correspondence of the core of the negative carbon-isotope excursion, temperature shows a maximum. This interval coincides with the most intense volcanic phase, as suggested by biomarkers, calcareous nannofossils and Os-isotope (MÉHAY et alii, 2009; TEJADA et alii, 2009; ERBA et alii., 2010; BOTTINI et alii., in press). This correspondence is suggestive of a (super)greenhouse climate triggered by excess volcanogenic CO2. Our data indicate that the beginning of the prolonged volcanic phase during OAE 1a coincides with warmest temperatures. The end of global anoxia is paralleled by a cooling episode which slightly postpone the end of the major OJP volcanism. High resolution analyses allow the identification of rapid cooling and warming during OAE 1a. Warm conditions in the early part of OAE 1a are interrupted by a brief (~35 ky) cooling interlude traced either by oxygen isotopes and calcareous nannofossils. This temperature change follows a ~100 kyrs-long interval characterized by accelerated continental weathering rates (BOTTINI et alii., in press). Arguably, warming at OAE 1a onset promoted methane hydrate dissociation (also suggested by carbon isotopes and biomarkers analyses; MEHAY et alii., 2009), which was perhaps instrumental in triggering continental weathering. Subsequent CO2 draw down, possibly during OJP quiescence, might explain the brief cooling interlude annihilated by warmest temperatures coeval with the onset of OJP paroxysmal phase. In the second part of OAE 1a two more cooling events (around segments C4 and C6 of the carbon-isotope curve) sandwich an interval of intermediate and fluctuating temperatures. Major and minor cooling episodes correlate with high TOC content, suggesting that burial of organic matter acted as storage of excess CO2, thus temporarily mitigating greenhouse conditions, although under persisting OJP activity. The data collected combined with an improved chronology show that volcanism of OJP caused general global warming. The excess burial of organic matter acted as an additional and alternative process influencing climate change. The micropaleontological data presented confirm and provide evidence for even more temperature fluctuations during OAE1a, although under persisting OJP volcanism. This observation may imply that volcanism was not the only factor influencing climate. The ocean/atmosphere system during OAE 1a was extremely dynamic and affected by a series of complex processes. In fact, positive and negative feedbacks alternated over 1.2 My and controlled climate variability.

Warming and cooling episodes across the early Aptian Oceanic Anoxic Event 1a: the role of pCO2 on climate changes / C. Bottini, E. Erba, D. Tiraboschi. - In: RENDICONTI ONLINE DELLA SOCIETÀ GEOLOGICA ITALIANA. - ISSN 2035-8008. - 21:2(2012 Sep), pp. 985-985. ((Intervento presentato al 86. convegno Congresso Nazionale della Società Geologica Italiana tenutosi a Arcavacata di Rende (Cz) nel 2012.

Warming and cooling episodes across the early Aptian Oceanic Anoxic Event 1a: the role of pCO2 on climate changes

C. Bottini
Primo
;
E. Erba
Secondo
;
D. Tiraboschi
Ultimo
2012

Abstract

The early Aptian Oceanic Anoxic Event 1a (OAE 1a: ~120 Ma) is a global phenomenon of organic matter burial in oxygen-depleted oceans. Volcanism, associated with the emplacement of the Ontong Java Plateau (OJP), is thought to be the main triggering mechanism for global anoxia, ocean acidification and greenhouse conditions (e.g. ERBA, 1994; LARSON & ERBA, 1999; JONES & JENKYNS, 2001; MÉHAY et alii, 2009; TEJADA et alii, 2009; BOTTINI et alii., in press). Independent studies on TEX86, sporomorphs and oxygen isotopes (e.g. MENEGATTI et alii., 1998; DUMITRESCU et alii., 2006; ANDO et alii, 2008; KUHNT et alii., 2011; MILLAN et alii., 2011; KELLER et alii., 2011) provided evidence for climate instability during OAE 1a. Nevertheless, the direct/indirect role of OJP volcanism, weathering rates and pCO2 on climate changes has not been established. We have performed a high-resolution integrated nannofossil-geochemical investigation of OAE 1a in three sites: Cismon core (Italian Southern Alps), Piobbico core (Umbria-Marche basin, Italy) and DSDP Site 463 Mid-Pacific Mountains. The existing stratigraphic framework for the three sites and available cyclochronology for the Cismon core (MALINVERNO et alii., 2010) allows high-resolution dating of biotic and environmental fluctuations. The nannofossil Temperature Index correlated with the oxygen-isotope record has revealed systematic and synchronous changes in the three sites. A warming pulse is registered prior to the onset of OAE 1a and corresponds to a prominent volcanic phase documented in the Os-isotope records (TEJADA et alii., 2009; BOTTINI et alii., in press). During OAE 1a, in correspondence of the core of the negative carbon-isotope excursion, temperature shows a maximum. This interval coincides with the most intense volcanic phase, as suggested by biomarkers, calcareous nannofossils and Os-isotope (MÉHAY et alii, 2009; TEJADA et alii, 2009; ERBA et alii., 2010; BOTTINI et alii., in press). This correspondence is suggestive of a (super)greenhouse climate triggered by excess volcanogenic CO2. Our data indicate that the beginning of the prolonged volcanic phase during OAE 1a coincides with warmest temperatures. The end of global anoxia is paralleled by a cooling episode which slightly postpone the end of the major OJP volcanism. High resolution analyses allow the identification of rapid cooling and warming during OAE 1a. Warm conditions in the early part of OAE 1a are interrupted by a brief (~35 ky) cooling interlude traced either by oxygen isotopes and calcareous nannofossils. This temperature change follows a ~100 kyrs-long interval characterized by accelerated continental weathering rates (BOTTINI et alii., in press). Arguably, warming at OAE 1a onset promoted methane hydrate dissociation (also suggested by carbon isotopes and biomarkers analyses; MEHAY et alii., 2009), which was perhaps instrumental in triggering continental weathering. Subsequent CO2 draw down, possibly during OJP quiescence, might explain the brief cooling interlude annihilated by warmest temperatures coeval with the onset of OJP paroxysmal phase. In the second part of OAE 1a two more cooling events (around segments C4 and C6 of the carbon-isotope curve) sandwich an interval of intermediate and fluctuating temperatures. Major and minor cooling episodes correlate with high TOC content, suggesting that burial of organic matter acted as storage of excess CO2, thus temporarily mitigating greenhouse conditions, although under persisting OJP activity. The data collected combined with an improved chronology show that volcanism of OJP caused general global warming. The excess burial of organic matter acted as an additional and alternative process influencing climate change. The micropaleontological data presented confirm and provide evidence for even more temperature fluctuations during OAE1a, although under persisting OJP volcanism. This observation may imply that volcanism was not the only factor influencing climate. The ocean/atmosphere system during OAE 1a was extremely dynamic and affected by a series of complex processes. In fact, positive and negative feedbacks alternated over 1.2 My and controlled climate variability.
Calcareous nannofossils; Climate changes; Early Cretaceous; Oceanic Anoxic Event 1a; Oxygen isotopes
Settore GEO/01 - Paleontologia e Paleoecologia
set-2012
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/2434/223081
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