Abstract Details
(2020) Berner Lecture: How Will Anthropogenic CO2 Affect Shallow Water Calcium Carbonate Sediment Dissolution?
Andersson A
https://doi.org/10.46427/gold2020.63
12c: Plenary Hall, Wednesday 24th June 22:00 - 22:03
Andreas Andersson
Listed below are questions that have been submitted by the community that the author will try and cover in their presentation. To submit a question, ensure you are signed in to the website. Authors or session conveners approve questions before they are displayed here.
Submitted by Christopher Sabine on Tuesday 16th June 19:34
Andreas, very nice talk. You focused on the high-mag calcites since they are the most soluble, but what are their typical concentrations in sediments? Is there a possibility of dissolving all the magnesian calcites in some locations and would the dissolution stop at that point until the solubility of aragonite is reached in the pore waters?
Thanks Chris! In typical coral reef environments Mg-calcites make up 25-30% of the sediments with an average Mg composition of 13-15 mol% MgCO3. There are certainly exceptions to these generalizations and the content and composition may also vary between different habitats. Several studies have proposed that pore water exists in a metastable equilibrium with the most soluble bulk mineral phase present in sediments and that dissolution follows a sequence according to mineral stability (e.g., Chave 1962, Schmalz and Chave, 1963, Neumann 1965; Morse et al. 2006). In such a scenario and assuming distinct solubility differences, one would expect dissolution of one phase before the next most soluble phase started to dissolve (see Fig. 5 in Morse et al. 2006, GCA). In reality, solubilities follow a continuum that are influenced by a range of factors including microarchitecture, impurities, surface coatings etc. I see no possibility of dissolving all the Mg-calcite owing to anthropogenic CO2 on timescales of centuries, but there could perhaps be changes to the composition of the most soluble bulk mineral phase. However, one also has to consider how the biogenic production of these mineral phases will be affected.
Andreas, very nice talk. You focused on the high-mag calcites since they are the most soluble, but what are their typical concentrations in sediments? Is there a possibility of dissolving all the magnesian calcites in some locations and would the dissolution stop at that point until the solubility of aragonite is reached in the pore waters?
Thanks Chris! In typical coral reef environments Mg-calcites make up 25-30% of the sediments with an average Mg composition of 13-15 mol% MgCO3. There are certainly exceptions to these generalizations and the content and composition may also vary between different habitats. Several studies have proposed that pore water exists in a metastable equilibrium with the most soluble bulk mineral phase present in sediments and that dissolution follows a sequence according to mineral stability (e.g., Chave 1962, Schmalz and Chave, 1963, Neumann 1965; Morse et al. 2006). In such a scenario and assuming distinct solubility differences, one would expect dissolution of one phase before the next most soluble phase started to dissolve (see Fig. 5 in Morse et al. 2006, GCA). In reality, solubilities follow a continuum that are influenced by a range of factors including microarchitecture, impurities, surface coatings etc. I see no possibility of dissolving all the Mg-calcite owing to anthropogenic CO2 on timescales of centuries, but there could perhaps be changes to the composition of the most soluble bulk mineral phase. However, one also has to consider how the biogenic production of these mineral phases will be affected.
Submitted by Maria Dittrich on Tuesday 23rd June 22:30
Congrats to this prestigious medal! Very nice talk. You said (and I agree) that caco3 dissolution is driven mostly by aerobic processes. My question is what will happen in case the amount of organic matter will increase drastically, so anaerobic mineralization will also take place, so CaCO3 dissolution may decline and be preserved?
Thank you Maria! This is an excellent question and I think there are a couple of ways we can think about it. At the community to ecosystem scales, reef environments and habitats affected by significant organic matter inputs (e.g., Kaneohe Bay, Hawaii; some enclosed environments in Palau; Bocas del Toro, Panama, etc) typically demonstrate lower surface seawater aragonite saturation state and have lower net ecosystem calcification rates than areas with less organic inputs. This can most likely be attributed to higher dissolution rates, but the details remain to be shown. However, at the site specific scale, increased organic matter and anaerobic mineralization could promote supersaturation and precipitation and/or recrystallization of CaCO3 that may compensate for part of the dissolution, but it depends on a number of factors including the extent of sulfate reduction (most important anaerobic process in these type of sediments), Fe availability and oxygen supply via advection, bioturbation or similar that could reoxidize H2S and drive additional dissolution. Lynn Walter and others did some wonderful work on this topic, but I think there is still more to be done. In particular since the organic matter loading to reef environments is something that potentially can be mitigated at the local scale.
Congrats to this prestigious medal! Very nice talk. You said (and I agree) that caco3 dissolution is driven mostly by aerobic processes. My question is what will happen in case the amount of organic matter will increase drastically, so anaerobic mineralization will also take place, so CaCO3 dissolution may decline and be preserved?
Thank you Maria! This is an excellent question and I think there are a couple of ways we can think about it. At the community to ecosystem scales, reef environments and habitats affected by significant organic matter inputs (e.g., Kaneohe Bay, Hawaii; some enclosed environments in Palau; Bocas del Toro, Panama, etc) typically demonstrate lower surface seawater aragonite saturation state and have lower net ecosystem calcification rates than areas with less organic inputs. This can most likely be attributed to higher dissolution rates, but the details remain to be shown. However, at the site specific scale, increased organic matter and anaerobic mineralization could promote supersaturation and precipitation and/or recrystallization of CaCO3 that may compensate for part of the dissolution, but it depends on a number of factors including the extent of sulfate reduction (most important anaerobic process in these type of sediments), Fe availability and oxygen supply via advection, bioturbation or similar that could reoxidize H2S and drive additional dissolution. Lynn Walter and others did some wonderful work on this topic, but I think there is still more to be done. In particular since the organic matter loading to reef environments is something that potentially can be mitigated at the local scale.
Submitted by Mario Goncalves on Wednesday 24th June 12:13
Great talk. My question is a bit aside as a curious question that bothers my mind but I never found any clear answer about that. I happen to have near me and during my undergraduate field work, prominent calcareous reef formations of end Cretaceous that, as I believe, is a feature that is common in many carbonate platforms of that age in many places of the World (at least in the Thetys ocean). Given that, although being estimates and inferences, the pCO2 of the atmosphere at the time was much greater than today, is there any evidence of these aspects of increasing calcium carboante dissolution affecting these reefs that can be inferred to have occurred at the time? Another observation, that I am not sure of how it may reflect the actual environment at the time, the effects of a higher pCO2 seems to have had a lesser impact on reef formation in the Cretaceous than it is antecipated in the present situation. It's also true that many of those organisms are now extinct, as the Rudists. Thanks!
Thank you Mario. It is a good question. It is insufficient to consider only one carbonate chemistry parameter (in this case pCO2) to assess the oceans favorability for CaCO3 production and preservation. The ocean chemistry was very different during the Cretaceous compared to the present. For example, the calcium concentration was probably much higher. Consequently, even though both pCO2 and pH were higher and lower, respectively, the saturation states with respect to carbonate minerals were much higher compared to the present, thus favoring production and preservation of CaCO3. Nonetheless, we have to be careful comparing long term steady states with the current short term perturbation. Honisch et al. (2012, Science 335, 1058) and references therein are good starts if you want to dwell in to this deeper.
Great talk. My question is a bit aside as a curious question that bothers my mind but I never found any clear answer about that. I happen to have near me and during my undergraduate field work, prominent calcareous reef formations of end Cretaceous that, as I believe, is a feature that is common in many carbonate platforms of that age in many places of the World (at least in the Thetys ocean). Given that, although being estimates and inferences, the pCO2 of the atmosphere at the time was much greater than today, is there any evidence of these aspects of increasing calcium carboante dissolution affecting these reefs that can be inferred to have occurred at the time? Another observation, that I am not sure of how it may reflect the actual environment at the time, the effects of a higher pCO2 seems to have had a lesser impact on reef formation in the Cretaceous than it is antecipated in the present situation. It's also true that many of those organisms are now extinct, as the Rudists. Thanks!
Thank you Mario. It is a good question. It is insufficient to consider only one carbonate chemistry parameter (in this case pCO2) to assess the oceans favorability for CaCO3 production and preservation. The ocean chemistry was very different during the Cretaceous compared to the present. For example, the calcium concentration was probably much higher. Consequently, even though both pCO2 and pH were higher and lower, respectively, the saturation states with respect to carbonate minerals were much higher compared to the present, thus favoring production and preservation of CaCO3. Nonetheless, we have to be careful comparing long term steady states with the current short term perturbation. Honisch et al. (2012, Science 335, 1058) and references therein are good starts if you want to dwell in to this deeper.
Submitted by Pierre Regnier on Wednesday 24th June 19:15
Hi Andreas, Congrats for this nice award and the way you framed your own research in the footsteps of Bob's career. I was ready to ask a question but then realized it was exactly the same as the one from Maria above ! Thus, I have one which is slightly disconnected from your talk but for which I would like to have your opinion as a reef expert (today or any other day). About 5 years ago, Smith & MaKenzie (2015) published a paper on net calcification globally and revised upwards the contribution from reef ecosystems (mostly because they took into account the surface area of submerged reefs in their upscaling). To me this makes sense, but they then reach on total particulate inorganic carbon burial (at least 0.25 PgC yr-1, with at least 50 % from banks and reefs) that significantly exceeds estimates of carbonate dissolution through weathering on land (about 0.1-0.15 PgC yr-1,), leading to a disequilibrium in the (pre-industrial) global C budget. Do you have any views on this ?
Thanks Pierre! My views are that I probably shouldn't criticize my Ph.D. advisor ;) Regardless, I don't have a strong view as I haven't thought about global budgets for awhile, but the pre-industrial marine CaCO3 cycle was probably not in a steady state owing to the sea level rise and flooding of the continental shelves since the LGM as proposed by Milliman. Whether the imbalance matches those estimates or not is a different question. Sorry I don't have a more definite answer at this time.
Hi Andreas, Congrats for this nice award and the way you framed your own research in the footsteps of Bob's career. I was ready to ask a question but then realized it was exactly the same as the one from Maria above ! Thus, I have one which is slightly disconnected from your talk but for which I would like to have your opinion as a reef expert (today or any other day). About 5 years ago, Smith & MaKenzie (2015) published a paper on net calcification globally and revised upwards the contribution from reef ecosystems (mostly because they took into account the surface area of submerged reefs in their upscaling). To me this makes sense, but they then reach on total particulate inorganic carbon burial (at least 0.25 PgC yr-1, with at least 50 % from banks and reefs) that significantly exceeds estimates of carbonate dissolution through weathering on land (about 0.1-0.15 PgC yr-1,), leading to a disequilibrium in the (pre-industrial) global C budget. Do you have any views on this ?
Thanks Pierre! My views are that I probably shouldn't criticize my Ph.D. advisor ;) Regardless, I don't have a strong view as I haven't thought about global budgets for awhile, but the pre-industrial marine CaCO3 cycle was probably not in a steady state owing to the sea level rise and flooding of the continental shelves since the LGM as proposed by Milliman. Whether the imbalance matches those estimates or not is a different question. Sorry I don't have a more definite answer at this time.
Submitted by Anne Laura Kruijt on Friday 26th June 15:03
Dear mr Andersson, thanks for the talk! At some point you mention the box-model you devised to study the effects of CO2 increase on coastal zone carbon processes and CaCO3 dissolution. This model is a 'global' box model if I understand correctly, and I was wondering how well this represents the processes occuring in coral reefs. Are there processes imaginable that could be of significant importance to the relationship between anthropogenic CO2, coastal water pH, organic matter decomposition and CaCO3 dissolution but are not well captured by a global model (e.g. latitudinal temperature differences, shelf area, local favourable conditions for organic matter production due to e.g. nutrient loading…)? And if so, are efforts being made at testing the importance of these more local conditions? Is the relationship between increasing antropogenic CO2 and
Thank you for your question Anne. The global model does not attempt to represent local conditions which varies from place to place but rather an average. All the things you list can affect the output but not necessarily the conclusions. The model use sensitivity analyses of different processes and parameterizations to see how they affect the output and whether they are important or not. For example, to assess the buffering effect question, the residence time is important to assess whether alkalinity accumulates in the system or not (see for example Morse et al., 2006, GCA, and Andersson and Mackenzie 2012, Biogeoscience). Some efforts were made for coral reef specific scenarios (see Andersson et al. 2005, AJS) but this model was essentially retired after that as my work transitioned to more field based activities.
Dear mr Andersson, thanks for the talk! At some point you mention the box-model you devised to study the effects of CO2 increase on coastal zone carbon processes and CaCO3 dissolution. This model is a 'global' box model if I understand correctly, and I was wondering how well this represents the processes occuring in coral reefs. Are there processes imaginable that could be of significant importance to the relationship between anthropogenic CO2, coastal water pH, organic matter decomposition and CaCO3 dissolution but are not well captured by a global model (e.g. latitudinal temperature differences, shelf area, local favourable conditions for organic matter production due to e.g. nutrient loading…)? And if so, are efforts being made at testing the importance of these more local conditions? Is the relationship between increasing antropogenic CO2 and
Thank you for your question Anne. The global model does not attempt to represent local conditions which varies from place to place but rather an average. All the things you list can affect the output but not necessarily the conclusions. The model use sensitivity analyses of different processes and parameterizations to see how they affect the output and whether they are important or not. For example, to assess the buffering effect question, the residence time is important to assess whether alkalinity accumulates in the system or not (see for example Morse et al., 2006, GCA, and Andersson and Mackenzie 2012, Biogeoscience). Some efforts were made for coral reef specific scenarios (see Andersson et al. 2005, AJS) but this model was essentially retired after that as my work transitioned to more field based activities.
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