Overview:
This manuscript presents data from studies into the sediment chemistry from a series of stations in the Baltic Sea with a focus on Mn and its potential as a tracer of oxygenation events in the anoxic deep waters. The experimental approach is very strong, well described, and state of the art and the article is well written to the point and concise. The authors interpret their dataset in terms of differing modes for Mn carbonate formation in the sediments in response to periodic injections of oxygenated waters into anoxic deep waters. In the Gotland deep they suggest that MnCO3 previously formed from MnO2 that was precipitated following North Sea inflows, but now is absent as the anoxia is now basin wide. While in the Landsort Deep, MnCO3 and MnS form independent of inflow events as there is a deep pore water source of reduced Mn. These findings have important implications in terms of paleo reconstructions of oxygenation events. However at present the paper lacks an appreciation of the time scales involved nor the potential for other controlling processes (e.g. pyrite formation and/or basin shape/size) and how this impacts the Mn inventories and concentrations in the bottom waters and porewaters. There is a frequent referral to a potential recent change in that the generation of H2S after an oxygenation event is faster now but no explanation is given of how this can occur. Waht would be most useful however is some sort of description or evaluation (model or otherwise) of what the tipping point is for a record of an oxygenation event to be recorded. For instance if there is a short interval between oxygen events is it recorded as one or two events, alternatively if there has been no event for several years and sulfide has built up appreciably, how big an event has to occur for it to be recorded in say either the Landsort or Gotland basins? Overall though this manuscript should be acceptable after moderate revision as outlined below.
General Comments:
Timescales of response to an oxygenation event:
One aspect that is poorly explored in the manuscript at present is how quickly the system responds to an oxygenation event and how quickly does it return to the previous state. Laboratory data indicates that MnS is apparently oxidized rapidly when re-suspended in oxygenated seawater (Simpson et al., 1998; Zhang and Millero, 1994) and may form MnO2 or even Mn(III) species. Simple abiotic oxidation of Mn(II) is very slow (von Langen et al., 1997) but is accelerated by bacteria (de Vrind et al., 1986; Sujith and Bharathi, 2011; Tebo and Emerson, 1986; Tebo, 1991; Tebo et al., 2005). There are a also works that look at the kinetics of reduction of MnO2 by H2S (Yao and Millero, 1993) and this information would be useful for looking at the timescale that is involved on that side of the redox cycle.
Reactive Mn, Mn(III) in the Baltic:
A difference in reactive Mn between the Gotland deep and Landsort deep has been commented on recently in another work (Dellwig et al., 2012). In their study they found almost no reactive Mn, thought to be Mn(III) species, in the Gotland deep. While in the Landsort deep there was a strong gradient of increasing reactive Mn towards the central part of the basin. These authors suggested lateral intrusions of oxygen into Gotland deep prevented the formation of stable suboxic zone where reactive Mn could accumulate. It would be useful to discuss then the potential for reactive Mn to impact the formation of MnCO3 etc as presumably an oxygenation event may lead to an increase in Mn(III) within the sediments (Luther III et al.; Madison et al., 2011; Oldham et al.; Trouwborst et al., 2006) that may be supported by organic complexing agents found there.
It should also be noted that there is now considerable Mn data from the Baltic (Dellwig et al., 2012; Kashiwabara et al., 2009; Neretin et al., 2003; Pakhomova and Yakushev, 2013; Pohl and Hennings, 2005; Yakushev et al., 2009) which illustrate that Mn in the anoxic waters in the Baltic can vary appreciably due to the oxygenation events and this needs to be taken into account. The recent work by Pakhomova and Yakushev (2013) is a good place to start as they provide a minor compilation of values and a modelling effort. In this context the recent works on Mn cycling at the redoxcline in the Baltic (Dellwig et al., 2012; Schnetger and Dellwig, 2012) should also be included in the discussion as during an oxygenation event in the deep waters the same sub-oxic chemistry will be brought into play in the sediments.
Mn incorporation into pyrite:
One aspect of Mn chemistry in anoxic environments that appears to be under explored here but could be a key pathway is the earlier finding that dissolved Mn can adsorb onto FeS phases and be pyritized (Morse and Luther III, 1999). These authors found that Mn was more readily incorporated into pyrite when the degree of pyritization (pyrite Fe/(pyrite Fe + reactive Fe) was above 40%. Thus the coexistence of Mn and S in the sediments may also be related to FeS availability and not MnS formation per se. In this case the ratio of Fe to Mn, the degree of pyritization, as well as the sulfide concentration would be important. Pyrite has already been shown to be an important species for transferring Fe to the sediments in the Gotland deep (Fehr et al., 2010). Information on the pyrite and Fe(II) content in the sediments/porewaters would also then help to gauge if this was having an impact on the Mn cycle.
The age of Baltic seawater and mixing in the Gotland Basin:
One of the major themes of this paper is the influx of oxygenated water into the different basins of the Baltic. It would therefore be of interest to link the present work with some recent physical oceanographic studies of the water mass age in the Baltic (Meier, 2005) and the Baltic sea mixing experiment BATRE (Holtermann and Umlauf, 2012; Holtermann et al., 2012). In the latter an inert tracer (SF5CF3) was injected into the Gotland Basin at 200 m depth and followed over 18 months or so. BATRE is useful in the present context as it points to the role of typography in the mixing of the bottom waters in this region and to the potential response time of an oxygenation event.
Mn carbonate formation and solubility:
There are a number of other recent works that have looked at the formation and solubility of Mn carbonates in seawater (Jensen et al., 2002; Johnson, 1982; Luo and Millero, 2003; Neher-Neumann, 1994; Radha and Navrotsky, 2014; Wartel et al., 1990). Note that there are also papers that deal with the transformation of aragonite/calcite to Mn carbonate (Böttcher, 1997; Böttcher, 1998) and this pathway should be mentioned also in the discussion along with data on the carbonate system in the water column (Schneider et al., 2010; Ulfsbo et al., 2011). This would then help to make the case that MnCO3 formation is only occurring in the uppermost part of the sediments.
Molybdenum:
The conversion of molybdate to reduced molybdenum species is reasonably complex and depends on the sulfide concentration (Erickson and Helz, 2000; Wang et al., 2009; Wang et al., 2011), so it would be useful to include a line or two on this. Also it is worth pointing out the strong conservative behavior for Molybdenum in the Baltic (Prange and Kremling, 1985) as due to the low salinities encountered, there are strong vertical gradients in Molybdenum that are not found in other regions. This makes it harder to see small losses of Mo due to scavenging.
Specific Comments:
P3 Line 30: So how does this mechanism work is it that the pore water Mn2+ is lower as the reduction takes place in the water column?
P7 Line 20: Inclusion of the seawater value and error estimates for the flux would be useful to include either here or later in the manuscript as a check to the validity of the flux estimates.
P8 Line 18: Is the data on the saturation for FeS included here in the manuscript? As I could not find it listed in the main text.
P9 Line 4: It would be useful to also provide here the saturation status for FeS at these sites?
P10 Line 25: In terms of the key processes it would be helpful to state here that release of Mn from sediments with MnO2 present is driven by redox processes, while in the case of MnCO3 it is controlled by the concentration of the solutes and there is no apparent redox change for Mn.
P11 Line 2: Is there no Mn incorporation into pyrite occurring then?
P11 Line 26: Is this controlled by the high Mn in the overlying anoxic waters? If so how much of a reservoir effect is this?
P11 Line 28: When the oxygenated water comes in to the basin it will obviously take some time to oxidize the bulk of the Mn2+ (Yakushev et al., 2011) and to form that MnO2 that is deposited to the sediment so there is an important kinetic effect here. Also this thin layer of MnO2 is then presumably reduced at the sediment interface resulting in a locally high concentration of Mn2+ which leads to the formation of MnCO3. This all relies on slow diffusion and no resuspension of the Mn porewaters in the sediments.
P14 Line 24: Why does the sulfide return more rapidly? High overlying productivity driving more rapid deoxygenation?
P14 Line 32: I would agree with this as MnO2 in the water column most likely is associated with an organic layer which would reduce its ability to scavenge Molybdate. Most studies that have looked into the ability of MnO2 to scavenge molybdate do not consider the effect of organics (Kashiwabara et al., 2009).
P15 Line 24: This sentence needs some qualification as while there might be 8 µM Mn(II) in the bottom waters and shallowest porewaters this might still be insufficient to precipitate out MnCO3 if the carbonate concentration is too low (low pH and/or low alkalinity) (Böttcher, 1997; Böttcher, 1998). There would also presumably be a sulfide control on the Mn(II) concentration.
P15 Line 27: Is the expansion in terms of horizontal area and/or vertical extent? Is there any data that the inventory of Mn in the water column is increasing?
Table 2: It would be helpful here to also include the Mn(II) concentration in both the overlying water and at the top of the porewater profile to show that the flux is real. In this context if a dissolved Mn inventory for the anoxic waters was constructed it would allow a turnover time for this pool to be estimated using the fluxes listed here. This would help to answer the question of how long it takes to resupply the bottom waters after an oxygenation event.
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