Biotic Response of Plankton Communities to Middle to Late Miocene Monsoon Wind and Nutrient Flux Changes in the Oman Margin Upwelling Zone
Abstract. Understanding the behavior of past upwelling cells is paramount when assessing future climate changes. Our present understanding of nutrient fluxes throughout the world's oceans emphasizes the importance of intermediate waters transporting nutrients from the Antarctic divergence into the middle and lower latitudes. These nutrient-rich waters fuel productivity within wind-driven upwelling cells in all major oceans. One such upwelling cell is located along the Oman Margin in the Western Arabian Sea (WAS). Driven by cross-hemispheral winds, the WAS upwelling zone’s intense productivity led to the formation of one of the most extensive oxygen minimum zones known today.
In this study covering the Middle to Late Miocene at ODP Site 722, we investigate the inception of upwelling-derived primary productivity. We combine novel data with existing model- and data-based evidence, constraining the tectonic and atmospheric boundary conditions for an upwelling cell to exist in the region. With this research, we build upon the original planktonic foraminifer-based research by Dick Kroon in 1991 as part of his research based on the Ocean Drilling Project (ODP) LEG 117.
We show that monsoonal winds likely sustained upwelling since the emergence of the Arabian Peninsula after the Miocene Climatic Optimum (MCO) ~14 Ma, with fully monsoonal conditions occurring since the end of the Middle Miocene Climatic Transition (MMCT) ~13 Ma. However, changing nutrient fluxes through Antarctic Intermediate and sub-Antarctic Mode Waters (AAIW/SAMW) were only established by ~12 Ma. Rare occurrences of diatoms frustules correspond to the maximum abundances of Reticulofenestra haqii and Reticulofenestra antarctica, indicating higher upwelling-derived nutrient levels. By 11 Ma, diatom abundance increases significantly, leading to alternating diatom blooms and high-nutrient-adapted nannoplankton taxa. These changes in primary producers are also well reflected in geochemical proxies with increasing δ15Norg. values (> 6 ‰) and high organic carbon accumulation also confirm high productivity and beginning denitrification simultaneously.
Our multi-proxy-based evaluation of Site 722B primary producers thus indicates a stepwise evolution of productivity in the western Arabian Sea related to the intensity of upwelling and forcing SAM dynamics throughout the Middle to Late Miocene. The absence of full correspondence with existing deep marine climate records also suggests that local processes, such as lateral nutrient transport, likely played an important role in modulating productivity in the western Arabian Sea. Finally, we show that using a multi-proxy record provides novel insights into how fossil plankton responded to changing nutrient conditions through time in a monsoon-wind-driven upwelling zone.
Gerald Auer et al.
Status: final response (author comments only)
- RC1: 'Comment on cp-2023-14', Anonymous Referee #1, 21 Apr 2023
- RC2: 'Comment on cp-2023-14', Anonymous Referee #2, 08 May 2023
Gerald Auer et al.
Global Ocean Colour (Copernicus-GlobColour), Bio-Geo-Chemical, L4 (monthly and interpolated) from Satellite Observations (1997-ongoing) https://doi.org/10.48670/moi-00281
Gerald Auer et al.
Viewed (geographical distribution)
The present contribution traces the evolution of the upwelling system in the NW Indian Ocean (and its atmospheric and oceanic drivers) over the time interval from 15 to 9 million years ago (mid- Miocene) and is based on data raised from sediment cores of ODP Site 722 on Owen Ridge. Counts of calcareous nannofossil taxa, siliceous fragments and planktonic foraminifers are presented in context with data on sedimentology, geochemistry and isotopic composition of the sediment sequence. The new microfossil assemblage data are interpreted as indicators of nutrient conditions in the mixed layer and in the thermocline, and the record is proposed to illustrate interactions between different plankton groups and varying nutrient levels and ratios. The data and interpretations of local developments are then discussed in perspective of regional and global changes in the coupled ocean-atmosphere system, which at that time experienced major reorganisation in response to tectonic processes and opening/closing of ocean gateways.
Novelty of the manuscript is in the statistical investigation of quantitative nannofossil data in 71 samples (for nannofossils and siliceous debris) and planktonic foraminifers in 28 samples and their implications for reconstruction of trophic state, nutrient limitations, and paleoproductivity in the mixed ocean surface and thermocline. The statistical analysis of these data yields four “taphogroups” (plus subgroups) that are proposed to be indicative of specific local surface water conditions, mainly of nutrient concentrations and nutrient ratios, and are employed to track the evolution of conditions at Site 722 through time. This reviewer is not qualified to evaluate whether the approach of using size- and morphology-based traits as expressions of nutrient regimes is standard practise, particularly in light of the stated “weak support for individual clusters reflecting the overall strong similarities in the assemblage composition of the studied samples” (line 212). The abstract of the Paasche (2010) publication on “Roles of nitrogen and phosphorus in coccolith formation in Emiliania huxleyi" suggests that reactions of this particular modern prymnesiophyte to nutrient limitations are much less straightforward than stated here.
Ancillary proxies, such as XRF data (dust and Mn), carbonate-C and organic-C, delta15N, have previously been published in Bialik et al. ( 2020). Why these are suitable proxies is unclear, because the introduction is incomplete and sections of text appear to be out of sequence and/or truncated in the presentation of proxies used to track wind, upwelling and OMZ (lines 93-101). In many cases the authors use these ancillary proxies to bolster their arguments based on the nannofossil assemblage, which occasionally results in circular reasoning.
As a main conclusion, the authors propose that the atmospheric (SW monsoon) driver and an incipient oxygen minimum zone existed in the western Arabian Sea, but that a one-million-year lag in productivity and community response to upwelling marks a teleconnection to southern hemisphere intermediate water formation: Only after intermediate waters formed in the Antarctic Convergence imported new nutrients into the thermocline of the northern Arabian Sea did the upwelling elicit a typical high-productivity response in the plankton community. Furthermore, they propose that changing nutrient ratios (caused by variable denitrification in a variable oxygen minimum zone) subsequently dictated changing patterns of planktonic ecosystems at continued high nutrient supply.
The manuscript thus offers interesting observational evidence of previously published modeling results and is an extension of a previous publication of some of the authors. The positive view is dampened somewhat by the (frequently unreflected, sometimes counterfactual and occasionally contradictory) rendition or reiteration of statements that are at least worthy of discussion. This particularly concerns the interactions between atmospheric drivers, water mass distributions and their nutrients, and dynamics of the oxygen minimum zone in the various subchapters of the discussion and the discussion of global implications. These parts of the manuscript definitely need careful scrutiny.
The manuscript fits the scope of the journal, in particular in a special issue dedicated to Dick Kroon, who has laid grounds for the reconstruction of the monsoonal upwelling in the Arabian Sea. It is reasonably well written, but comparatively long for what the authors have to say, and the list of references is very long indeed. In a revision, text should be edited with care and preferably by a native speaker. Some passages of text on intricacies of placolith morphologies should be omitted. The continual use of comparatives without reference to what it is compared to must be corrected. Figures are ok, but may possibly be condensed by plotting factor scores instead of single species´ abundances.
Detailed comments and questions:
Many of the data and many of the arguments in the discussion echo the Bialik et al. (2020; Paleoceanography and Paleoclimatology) publication: It is necessary to highlight the novelty of the present paper more clearly and succinctly.
A number of conclusions in that publication are taken for granted here, so that some of the questions below are addressed to the precursor publication. The following more or less relevant points came to my mind when reading the paper:
Detailed comments keyed to line numbers:
17 I find the statement in first sentence of the abstract difficult to understand. Why is that so?
20 In my understanding, upwelling cells are localized spots of high upwelling intensity caused by the interactions of wind and local topography. Many of these cells combined form upwelling systems. One is the WAS upwelling system, the only major western boundary upwelling system.
37 Concentration of TOC in the sediment says nothing about accumulation! Take care to not use these two terms synonymously….
54 Upwelling acts as both sink and source of CO2 to the atmosphere, and each system differs in the net balance.
200 does this mean number of nannos per g of CaCO3 or divided by CaCO3? Unusual annotation! Are two digits after the decimal within the confidence limits of your method?
231ff So, are the results statistically robust, or not?
236 …2, whereas
258 placoliths are the small plates, arent´t they? How can they proliferate?
263-266 How can a “highly productive open marine environment” be nitrogen limited? This means an excess of phosphate and at that point, nitrogen fixation should kick in to make up the deficit. Please explain.
268 elevated sources?
269 N-limited nutrient sources meaning low N:P ratios in upwelling thermocline water?
273 elevated nutrient levels compared to what? Frequently the comparative is used throughout the text without reference to what the comparison refers to.
277-278 The thermocline and nutricline coincide usually and there the nutrient levels are high at the base – that is why the plants are there in the first place even at low light levels (deep chlorophyll maximum) …..so, what does “elevated nutrient conditions” mean?
279 How does this setting differ from conditions of TG 1(b)?
286 concentrations, not accumulation rates!
287 Why should that high dust flux increase productivity? Or is that a consequence of enhanced ballasting and export flux?
290 more abundant than what?
294 How could such a nitrogen-excess situation arise? Very difficult to imagine at fixed N:P ratios in thermocline waters and active denitrification in the OMZ! It appears that you put a lot of trust in a limited set of culture data for recent N-cell clones of Emiliania huxleyi.
295 strong upwelling
299-301 weaker, stronger, higher than what?
304 P-limitation (see query above)
310 what is an active OMZ?
333 (and 344) more limited than what? If there is no upwelling and only low productivity, how would an OMZ form in the first place? What is the exact link between monsoon strength and the OMZ without an intermediate link created by organic matter rain rate and water mass residence time?
359 How and why is intensified upwelling linked to high delta15N?
390 what amplifies? Declining upwelling?
401 I may have missed an explanation, how upwelling intensity is recorded in delta15N values.
411 What is the role of the temperature gradient today? Is it the influence on the sea level pressure gradient? How is the deep-water temperature gradient (between which end members?) involved?
415 gradients, thereby
428 indicated by instead of related to?
437 I must have missed the link between Mn/Al ratios and productivity. OMZ intensity comes to mind, but how is that linked to low productivity in a low nutrient regime?
440 As a result of? Explain the link between SST, sea level and upwelling!
443 why poorly ventilated? In line 461 below you state that that increase/decrease is indication of a shallow and poorly vented thermocline - what changed?
464 this is not accumulation! You might have a lot of TOC raining down, but when it is diluted by a lot of dust, for example, TOC concentrations are low!
468 formation of nutrients? Are they formed, or are they not used up because of light limitation?
482 mineralizing primary producers – is that a commonly used term? Other people use that word for dissimilation of organic matter and nutrient release
505-514 This entire discussion is very difficult to follow and possibly not suitable here: You infer from a size shift in one genus that the nutrient regime changed, but then discount this explanation and invoke changing nutrient limitation, but do not state the nature of that limitation.
518 – 520 the concept of Mn-redirection was lost in the introduction. Do you talk about sediments, or water? Are high concentrations in sediments seen at the top and bottom of the OMZ where it intercepts the margin?
531 Shifts in nutrient saturation? I don´t think that you can saturate seawater in nutrients.
551 are you talking about N:P:Si ratios?
558 intermixing with
560 There is abundant literature on iron supply from continental margin sediments, particularly when they are situated in an OMZ
570 indicate a change in
575 quantity of nutrient enrichment?
579 Explain how that affects the northern AS (see above)! I am not entirely convinced that the record from the AS is compelling evidence….there may be other factors at play.
583 what is “nutrient rejuvenation”?
588 I am not sure I understand the argument for an increasing wind regime.
592 Explain how and why wind shear increases, then causes a global shift in ocean-atmosphere circulation, and deepens the thermocline. In my view, increasing wind shear causes open ocean upwelling and shallowing of the thermocline!
613 nutrient poor. But how then do you explain the OMZ that is apparently evident at that time?
624-626 According to You and Tomczak, 1993 and You, 1997, the upwelling taps essentially Northern Indian Ocean Central Water mixed with Red Sea/Persian Gulf waters.
635 That drop in SST is certainly not exclusively linked to a specific water mass, or is it? Not to enhanced upwelling?
662 what kind of shift? Excess phosphate? Less silicate?
666 you never refer to fluxes, but to concentrations. Would it not be simple to use your age model to actually calculate component fluxes from 722 GRAPE data?
667 delta13C is not shown in Fig. 3
669 which environmental stressors aside from nutrients?
References not cited in the manuscript:
You, Y. and Tomczak, M, 1993. Thermocline circulation and ventilation in the Indian Ocean derived from water mass analysis. DSR I, 40-1, 13-46.
You, Y., 1997. Seasonal variations of thermocline circulation and ventilation in the Indian Ocean. JGR, 102/C5, 10391-10422.