the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Mar 2023
Warming drove the Expansion of Marine Anoxia in the Equatorial Atlantic during the Cenomanian Leading up to Oceanic Anoxic Event 2
Mohd Al Farid Abraham
Bernhard David A. Naafs
Vittoria Lauretano
Fotis Sgouridis
Richard D. Pancost
Abstract. Oceanic Anoxic Event (OAE) 2 (~93.5 millions of years ago) is characterized by widespread marine anoxia and elevated burial rates of organic matter. However, the factors that led to this widespread marine deoxygenation and the possible link with climatic change remain debated. Here, we report long-term biomarker records of water column anoxia, water column and photic zone euxinia (PZE), and sea surface temperature (SST) from Demerara Rise in the equatorial Atlantic that span 3.8 million years of the late Cenomanian to Turonian, including OAE 2. We find that total organic carbon (TOC) contents are high but variable (0.41–17 wt. %) across the Cenomanian and increase with time. This long-term TOC increase coincides with a TEX86-derived SST increase from ~ 35 to 40 °C as well as the episodic occurrence of 28,30-dinorhopane (DNH) and lycopane, indicating warming and expansion of the oxygen minimum zone (OMZ) predating OAE 2. Water column euxinia persisted through much of the late Cenomanian, as indicated by the presence of C35 hopanoid thiophene, but only reached the photic zone during OAE 2, as indicated by the presence of isorenieratane. Using these biomarker records, we suggest that water column anoxia and euxinia in the equatorial Atlantic preceded OAE 2 and this deoxygenation was driven by global warming.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Journal article(s) based on this preprint
Mohd Al Farid Abraham et al.
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2023-260', Alexandre Pohl, 27 Apr 2023
- AC1: 'Reply on RC1', Mohd Al Farid Abraham, 22 Sep 2023
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AC3: 'Reply on RC1', Mohd Al Farid Abraham, 28 Sep 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-260/egusphere-2023-260-AC3-supplement.pdf
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RC2: 'Comment on egusphere-2023-260', Anonymous Referee #2, 03 Jul 2023
This paper looks at the organic geochemistry of the black shales deposited in the run-up to OAE 2 at Demerara Rise in the equatorial proto-Atlantic. Using a range of biomarkers the suthors plot the increase in deoxygenation that moved in concert with increasing temperature, as documented by TEX86 data. Although association does not prove cause and effect, the paleoceanographic model they suggest makes general sense and they are careful to look also at the paleotectonic context of their section in the light of the evolving South Atlantic, which could have impacted basin geometry and watermass stratification. An interesting highlight of the paper is the switch away from lycopane that is present in the upper Cenomanian to isorenieratane over the OAE 2 interval itself (Cenomanian–Turonian boundary), suggesting a change in the bacterioplankton consortium as photic-zone euxinic conditions took hold: presumably due to invasion by green sulfur bacteria.
Given the sampling density of this core, and the fact that the sedimentary material can move around, I wondered whether the samples giving new data could be accurately fixed in the stratigraphy and combined with pre-existing data. Do the authors have any feeling for this?
My main grouse in the account is the mixing between rock (or sediment) and time and between rock and process. You cannot sample an OAE or pass up into it: it is a phenomenon that leaves a distinctive record. You cannot have a TOC value for OAE 2! Use of the term ‘interval’, which can be applied to both sediment and time, can be helpful.
Geological narrative should not be in the present tense.
In-text references should be preferably ordered consistently by date
Details:
Line 23: I would hyphenate ‘water column’ where used as a compound noun–noun adjective
Line 27 and elsewhere: although the journal allows alphabetical in-text citations, I think that ordering by date is much to be preferred, as this technique indicates the academic trajectory of the point in question and gives credit where it is due. Referencing is date ordered in some places in the manuscript . . . . but see below (Line 31 and 34). Make consistent throughout the manuscript.
`line 31: references are not ordered by either alphabet or date!
Line 34: references are not ordered by either alphabet or date!
Line 41: Avoid beginning a sentence with an unqualified ‘This’ or ‘These’, which is often ambiguous. I suggest ‘These phenomena’
`line 76: I assume ‘sediment’ should be ‘samples’? And change ‘was’ to ‘were’
Line 77: better would be “Cenomanian to Turonian interval’
Line 79: here you are mixing rock and time. Unit IV is stratigraphically underlain by Albian phosphatic calcareous claystone
Line 83: there is confusion here between a phenomenon (such as an OAE) and sediment. You cannot sample an OAE, only its sedimentary record. Rephrase with something like ‘over the OAE 2 interval’. (‘Interval’ can be used for both rock and time and is a very useful term in this regard). I suggest changing ‘following’ (time word) to post-dating.
Line 84: you cannot identify OAE 2 at Site 1258 (it is long gone!) Change to ‘OAE 2 interval’
Line 101? We know that sediments can move position in a core, so conflating data sets fron samples taken years apart can be dangerous if correct stratigraphic order is not retained. How confident are the authors that all data are in correct stratigraphic order?
Line 117: better would be ‘. . . serves as a proxy for water-column anoxia. . . .’
Line 142: ‘during’ is a time word and you are describing a geochemical characteristic of a sediment sample. ‘ . . in the OAE 2 interval . . . ‘ would be better
Line 158” change to ‘water-column’ (with a hyphen)
Line 162: change to ‘up to the OAE 2 intervaL’
Line 166: change to ‘lower Cenomanian’: this is rock not time.
Line 167: water-column
Line 177: replace ‘ “Following OAE 2’ by ‘stratigraphically higher than the OAE 2 interval’ or similar
Line 178: change to ‘Upper Cretaceous’ – you are describing a feature of the sediment
Line 182: delete hyphen after ‘long-‘
Line 183” better would be: ‘culminating in the OAE 2 interval
Line 190:: hyphenate ‘bottom water’ used as a compound adjective
Line 191: better would be ‘ just below the onset level of OAE 2’ (to avoid mixing sediment with a phenomenon)
Line 194: replace ‘sometime’ with ‘in some cases’
Line 205” ‘over the OAE 2 interval’
Line 203: but presumably impinging on the sea floor if benthic foraminifera are affected?
Line 205: change to ‘This phenomenon has also been reported. . . .’
Line 211: change to ‘highest in the OAE 2 interval . . . ’
Line 211: change to ‘were driven’ – this is geological narrative
Line 213” ‘were replaced’
Line 218: ‘over the OAE 2 interval’ would be better
Line 219: water-column
Line 224: change ‘into’ to ‘during’
Line 225: water-column
Line 226: This process also gives rise. . . .
Line 230/231: change to : ‘water-column’
Line 234: change to: ‘these metal-isotope data by confirming . . .’
Line 234: water-column
Line 235: change ‘adding’ to ‘adds’
Line 235 Line 225: water-column
Line 240: delete hyphen from adverb to give ‘nearly closed’ – this is journal house style
Line 244: change to ‘This result. . . .
Line 245: change ‘of’ to ‘for’
Line 347: Line 225: water-column
Line 253: ‘Therefore, this result illustrates the occurrence of marine water-column anoxia .
Line 256: change to ‘propagated’. .’
Line 257: should this be ‘bottom-water anoxia’?
Line 257: change to ‘was linked’
Line 258: change to ‘was overridden’
Line 259: ‘watermass’ (one word)
Line 260: change to ‘in contrast to. . . .’
Line 264: should this be ‘deep-water convection’?
Line 266: water-column
Line 271: change to ‘during the Cenomanian. . . .’
Line 273: change to ‘during this time’ (not necessary to repeat ‘Cenomanian’
Line 284: change to ‘that persisted. . . .’
Line 287: change ‘’is not’ to ‘was not’
Line 289: change to ‘was related’
Line 292: water-column
Line 295: not clear what ‘these’ refers to – clarify
Line 298: water-column
Line 298/299: anoxia correlates with warming but association does not prove cause and effect
Line 300: water-column
Line 305: better would be ‘albeit modulated by climatic factors
579/580” Cross-plot and water-column (add hyhens)
Citation: https://doi.org/10.5194/egusphere-2023-260-RC2 - AC2: 'Reply on RC2', Mohd Al Farid Abraham, 22 Sep 2023
-
AC4: 'Reply on RC2', Mohd Al Farid Abraham, 28 Sep 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-260/egusphere-2023-260-AC4-supplement.pdf
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-260', Alexandre Pohl, 27 Apr 2023
- AC1: 'Reply on RC1', Mohd Al Farid Abraham, 22 Sep 2023
-
AC3: 'Reply on RC1', Mohd Al Farid Abraham, 28 Sep 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-260/egusphere-2023-260-AC3-supplement.pdf
-
RC2: 'Comment on egusphere-2023-260', Anonymous Referee #2, 03 Jul 2023
This paper looks at the organic geochemistry of the black shales deposited in the run-up to OAE 2 at Demerara Rise in the equatorial proto-Atlantic. Using a range of biomarkers the suthors plot the increase in deoxygenation that moved in concert with increasing temperature, as documented by TEX86 data. Although association does not prove cause and effect, the paleoceanographic model they suggest makes general sense and they are careful to look also at the paleotectonic context of their section in the light of the evolving South Atlantic, which could have impacted basin geometry and watermass stratification. An interesting highlight of the paper is the switch away from lycopane that is present in the upper Cenomanian to isorenieratane over the OAE 2 interval itself (Cenomanian–Turonian boundary), suggesting a change in the bacterioplankton consortium as photic-zone euxinic conditions took hold: presumably due to invasion by green sulfur bacteria.
Given the sampling density of this core, and the fact that the sedimentary material can move around, I wondered whether the samples giving new data could be accurately fixed in the stratigraphy and combined with pre-existing data. Do the authors have any feeling for this?
My main grouse in the account is the mixing between rock (or sediment) and time and between rock and process. You cannot sample an OAE or pass up into it: it is a phenomenon that leaves a distinctive record. You cannot have a TOC value for OAE 2! Use of the term ‘interval’, which can be applied to both sediment and time, can be helpful.
Geological narrative should not be in the present tense.
In-text references should be preferably ordered consistently by date
Details:
Line 23: I would hyphenate ‘water column’ where used as a compound noun–noun adjective
Line 27 and elsewhere: although the journal allows alphabetical in-text citations, I think that ordering by date is much to be preferred, as this technique indicates the academic trajectory of the point in question and gives credit where it is due. Referencing is date ordered in some places in the manuscript . . . . but see below (Line 31 and 34). Make consistent throughout the manuscript.
`line 31: references are not ordered by either alphabet or date!
Line 34: references are not ordered by either alphabet or date!
Line 41: Avoid beginning a sentence with an unqualified ‘This’ or ‘These’, which is often ambiguous. I suggest ‘These phenomena’
`line 76: I assume ‘sediment’ should be ‘samples’? And change ‘was’ to ‘were’
Line 77: better would be “Cenomanian to Turonian interval’
Line 79: here you are mixing rock and time. Unit IV is stratigraphically underlain by Albian phosphatic calcareous claystone
Line 83: there is confusion here between a phenomenon (such as an OAE) and sediment. You cannot sample an OAE, only its sedimentary record. Rephrase with something like ‘over the OAE 2 interval’. (‘Interval’ can be used for both rock and time and is a very useful term in this regard). I suggest changing ‘following’ (time word) to post-dating.
Line 84: you cannot identify OAE 2 at Site 1258 (it is long gone!) Change to ‘OAE 2 interval’
Line 101? We know that sediments can move position in a core, so conflating data sets fron samples taken years apart can be dangerous if correct stratigraphic order is not retained. How confident are the authors that all data are in correct stratigraphic order?
Line 117: better would be ‘. . . serves as a proxy for water-column anoxia. . . .’
Line 142: ‘during’ is a time word and you are describing a geochemical characteristic of a sediment sample. ‘ . . in the OAE 2 interval . . . ‘ would be better
Line 158” change to ‘water-column’ (with a hyphen)
Line 162: change to ‘up to the OAE 2 intervaL’
Line 166: change to ‘lower Cenomanian’: this is rock not time.
Line 167: water-column
Line 177: replace ‘ “Following OAE 2’ by ‘stratigraphically higher than the OAE 2 interval’ or similar
Line 178: change to ‘Upper Cretaceous’ – you are describing a feature of the sediment
Line 182: delete hyphen after ‘long-‘
Line 183” better would be: ‘culminating in the OAE 2 interval
Line 190:: hyphenate ‘bottom water’ used as a compound adjective
Line 191: better would be ‘ just below the onset level of OAE 2’ (to avoid mixing sediment with a phenomenon)
Line 194: replace ‘sometime’ with ‘in some cases’
Line 205” ‘over the OAE 2 interval’
Line 203: but presumably impinging on the sea floor if benthic foraminifera are affected?
Line 205: change to ‘This phenomenon has also been reported. . . .’
Line 211: change to ‘highest in the OAE 2 interval . . . ’
Line 211: change to ‘were driven’ – this is geological narrative
Line 213” ‘were replaced’
Line 218: ‘over the OAE 2 interval’ would be better
Line 219: water-column
Line 224: change ‘into’ to ‘during’
Line 225: water-column
Line 226: This process also gives rise. . . .
Line 230/231: change to : ‘water-column’
Line 234: change to: ‘these metal-isotope data by confirming . . .’
Line 234: water-column
Line 235: change ‘adding’ to ‘adds’
Line 235 Line 225: water-column
Line 240: delete hyphen from adverb to give ‘nearly closed’ – this is journal house style
Line 244: change to ‘This result. . . .
Line 245: change ‘of’ to ‘for’
Line 347: Line 225: water-column
Line 253: ‘Therefore, this result illustrates the occurrence of marine water-column anoxia .
Line 256: change to ‘propagated’. .’
Line 257: should this be ‘bottom-water anoxia’?
Line 257: change to ‘was linked’
Line 258: change to ‘was overridden’
Line 259: ‘watermass’ (one word)
Line 260: change to ‘in contrast to. . . .’
Line 264: should this be ‘deep-water convection’?
Line 266: water-column
Line 271: change to ‘during the Cenomanian. . . .’
Line 273: change to ‘during this time’ (not necessary to repeat ‘Cenomanian’
Line 284: change to ‘that persisted. . . .’
Line 287: change ‘’is not’ to ‘was not’
Line 289: change to ‘was related’
Line 292: water-column
Line 295: not clear what ‘these’ refers to – clarify
Line 298: water-column
Line 298/299: anoxia correlates with warming but association does not prove cause and effect
Line 300: water-column
Line 305: better would be ‘albeit modulated by climatic factors
579/580” Cross-plot and water-column (add hyhens)
Citation: https://doi.org/10.5194/egusphere-2023-260-RC2 - AC2: 'Reply on RC2', Mohd Al Farid Abraham, 22 Sep 2023
-
AC4: 'Reply on RC2', Mohd Al Farid Abraham, 28 Sep 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-260/egusphere-2023-260-AC4-supplement.pdf
Peer review completion
Journal article(s) based on this preprint
Mohd Al Farid Abraham et al.
Mohd Al Farid Abraham et al.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(1007 KB) - Metadata XML
-
Supplement
(403 KB) - BibTeX
- EndNote
- Final revised paper