Monsoonal forcing of calcification in the Arabian Sea

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Abstract

This paper summarizes our results on the changes in pelagic calcification and the standing stock of calcium carbonate associated with the SW Monsoon and NE Monsoon (cruises TN049 and TN053 of the U.S. JGOFS study, respectively) in the northern portion of the Arabian Sea. Mean calcification was ∼3X greater during the SW Monsoon than during the NE Monsoon. Calcification per coccolithophore was 7–10X higher, and the ratio of calcification to photosynthesis (C/P) was 40–45% higher during the SW Monsoon. The turnover time of PIC was not statistically different between the two cruises (∼4.5 d averaged over the euphotic zone). Turnover time of POC increased significantly between TN049 and TN053 (from ∼3 to 6 d over the euphotic zone). We discuss vertical sections of coccolithophore abundance, carbon standing stocks and carbon fixation. Coccolithophore calcification was usually about 1–5% of community photosynthesis. The ratio of calcification to photosynthesis spanned almost 2 orders of magnitude, and was not significantly different from the ratio of the PIC and POC standing stocks. We compare surface PIC and POC production rates to sediment trap fluxes from the same region.

Introduction

The JGOFS program is concerned with fluxes of carbon to the sediments and its prediction (Smith et al., 1998); much of the work in the JGOFS program has addressed the production, transformation and fate of organic carbon fractions. Our hypothesis is that the inorganic carbon flux from coccoliths is of equal significance to the organic flux in terms of carbon burial in sediments, and is of interest in terms of carbon removal on geological time-scales. This aspect of the carbon cycle is particularly interesting in the Arabian Sea due to its annual cycle of high productivity and high expected calcite fluxes.

Primary production rates from the coastal Arabian Sea can exceed 2 gC m−2 d−1 (with peaks exceeding 6 gC m−2 d−1; see Ryther and Menzel, 1965). On an annual basis though, rates of primary productivity in the north Arabian Sea are 5.3×1014 gC yr−1 (Qasim, 1982), which when averaged over its 1.5 million square km of area yields 340 gCm−2 yr−1. Since the total organic carbon flux at 3 km depth in the Western and Central Arabian Sea is 1.8 and 1.5 g m−2 yr−1, respectively (Nair et al., 1989) then on average, about 0.5% of the surface organic carbon productivity is thought to reach the sea floor. (Obviously, this figure is higher in upwelling centers then this areal average.) This argues for a total potential burial of organic carbon of about 2.65×1012 g C yr−1 in the Northern Arabian Sea. Sclater et al. (1977) provided estimates of the regional distribution of organic rich sediments in the Arabian Sea, including the strong increase in % organic carbon (to values>2% by weight) in surface sediments in the upwelling region off the coast of Oman.

Considerably less is known about global calcification, let alone calcification in the Arabian Sea. Globally, calcium carbonate sediments represent about a quarter of all marine sediments (Broecker and Peng, 1982). The current global CaCO3 production rate is about 0.6 Gtons C as CaCO3 yr−1 (Milliman, 1993; his Table 1). Annual production of CaCO3 represents only about 1–3% of the global marine organic carbon production (e.g., Milliman, 1993; Koblentz-Mishke et al., 1970; Berger, 1989; Longhurst et al., 1995), but due to extremely efficient remineralization of organic tissue by bacteria and higher organisms, CaCO3 dominates carbon burial in many regions. It is generally thought that CaCO3 represents a progressively larger fraction of the total carbon flux as particles sink (Westbroek et al., 1993).

For each mole of CaCO3 produced, 1 mole of CO2 is also released (Berger and Keir, 1984), which, for coccolithophores may be used in photosynthesis. Similarly, dissolution of the CaCO3 consumes CO2. Loss of Ca++ via calcification means that calcification affects alkalinity much more than organic carbon production does.2HCO3+Ca++CO2+H2O+CaCO3.Westbroek et al. (1994) pointed out that coupling of calcification and photosynthesis as well as respiration and CaCO3 dissolution means that calcification releases less CO2(aq) into the ocean and atmosphere than one might expect. To put the above stoichiometry into perspective, given a global calcification rate of 0.6 Gt C yr−1, there should be approximately an equimolar amount of CO2 produced each year. This represents ∼1/8 of the fossil fuel CO2 production, and is equal in magnitude to CO2 production associated with timber cutting and agricultural tilling of soils (Broecker and Peng, 1982). Calcium carbonate is produced in shallow waters by either coral reefs or Halimeda macrophytes, or in the plankton, by coccolithophores, foraminfera, and pteropods. Recent budgets suggest that calcification by shelf and open ocean plankton accounts for ∼2/3 of global calcification (Milliman, 1993).

Coccoliths comprise a significant portion of the CaCO3 content of pelagic sediments (Lohmann, 1908; Bramlette, 1958), and the distribution of many coccolithophores in surface waters matches their sediment distribution (McIntyre and Be, 1967). The global ratio of deposited CaCO3 to biologically fixed CaCO3 ranges from 14 to 55% (Broecker and Peng, 1982; Milliman, 1993). In terms of carbon actually reaching the ocean floor, the global sedimentation flux of inorganic carbon from coccoliths is about equal to the organic carbon flux. Considering carbon burial in the “geological archive”, the global CaCO3 flux dominates organic carbon by a factor of 6X (Westbroek et al., 1993).

Expected calcification in the Arabian Sea can be calculated using a combination of historical 14C measurements of primary production, and the global ratio of organic production to calcification (1 mol of calcium carbonate formed for every 4.5 mol of organic carbon; Broecker and Peng, 1982). We have measured this ratio at several sites in the North Atlantic Ocean and find that this ratio, although highly variable, is reasonable. Thus, if the average organic fixation in the surface Arabian Sea is 340 gC m−2 yr−1 (Qasim, 1982), then the expected calcite production rate should be ∼76 gC m−2 yr−1. Assuming a 6 month period of active growth, then this would give an integrated daily calcification rate of 414 mg C m−2 d−1.

One can cross-check the above calcification and photosynthesis estimates by using the equations of Broecker and Peng (1982, p. 474), and the GEOSECS data from station 416 in the Northern Arabian Sea. The value of net carbon fixation into calcium carbonate based on Eq. (9-4) of Broecker and Peng (1982) is 30.52 μmol C l−1 after correcting for density (sigma theta was about 26 for converting kg to liters). Organic carbon production is then ∼200 μmol C l−1 based on Eq. (9-5) of Broecker and Peng (1982). To convert these to rates, they can be multiplied by the average upwelling velocity of 1×10−3 cm s−1, found up to 400 km offshore (Smith and Bottero, 1977), with values three times that inside upwelling centers. Assuming that the bulk of the annual upwelling occurs for 3 months per year during the SW Monsoon at a rate of 1×10−3 cm s−1, results in a calcification rate of 0.32 gC m−2 d−1 (or ∼28.5 gC m−2 yr−1). Organic production during this period would be about 2.1 gC m−2 d−1 (or 189 gC m−2 yr−1). Thus, two radically different, indirect estimates of the daily calcification rate in the Arabian Sea agree within ∼25%.

The purpose of this work was to (1) measure directly calcification in the Arabian Sea and to compare these data with indirect estimates of calcification, (2) examine the variability of calcification over space and time, between the SW Monsoon and the early NE Monsoon, (3) calculate turnover of organic and inorganic carbon fractions, and (4) compare the direct estimates with sediment trap fluxes of calcium carbonate.

Section snippets

Cruise track

We participated on process cruises 4 (“TN049”; July 17–August 14, 1995) and 6 (“TN053”; October 24–November 25, 1995) of the US JGOFS Arabian Sea program. In the first cruise, we sampled 25 stations of the official US JGOFS “station line” plus two extra stations on the Omani shelf. The second cruise was the US JGOFS Bio-optics cruise where we sampled 14 stations on, and eight stations off, the US JGOFS station line. Process cruise six sampled Arabesque stations (UK JGOFS) within the Gulf of

Regional data comparison between SW and NE Monsoons

During the SW Monsoon, the depth of the 1% light level varied from 23–100 m over the study area while during the early NE Monsoon, the 1% light depths varied from 46–90 m. Surface mixing during the SW Monsoon also caused smaller vertical gradients in temperature. For example, the average euphotic zone temperature gradient during TN049 was −0.022°C m−1 (a negative value means the water was colder with depth; SD=0.066°C m−1; n=197) while during TN053 the average gradient was −0.051°C m−1 (SD=0.135; n

Productivity methods

Primary productivity, measured with the microdiffusion technique, gave higher values when water was incubated in SIS rather than IS conditions. There should be no bottle-related differences since bottles were identical for both incubations. The question is whether this difference was due to light, temperature or pressure. Pressure was probably not important, since the absolute differences between the two techniques actually decreased with increasing depth (data not shown). Temperature was

Summary

These data suggest that one of the more important sources of carbon to the sediments, coccolithophore calcification, shows strong covariation with bulk algal photosynthesis (usually calcification is ∼5% of total photosynthesis). Note, the ratio of photosynthesis to calcification was not 4.5 as we expected from previous calculations based on broad assumptions, but closer to 50. For the two time periods examined, the SW Monsoon clearly had more calcification, yet it was the more mixed period.

Acknowledgements

We are deeply indebted to the captain and crew of the R/V Thomas Thompson, whose expert ship handling made collection of these samples possible, often during difficult sea state. M. Bowen helped with logistical support in Oman. Dr. R. Barber, chief scientist (TN049), and Dr. J. Marra (TN053) kindly provided help with in situ incubations. We thank Dr. J. Dymond, Dr. S. Honjo, Dr. R. Barber, and Dr. J. Marra, for use of their respective data sets which were used for comparison with our results.

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      Few exceptions included two samples from the Arabian Sea (TTN-054 cruise) which were taken at 13 m and 20 m but still well within the surface mixed layer that extended to 40–60 m at these stations. Except for 15 POC samples from the Arabian Sea (Balch et al., 2000; Gundersen et al., 1998) and 2 samples from the MALINA cruise (Doxaran et al., 2012), the post-cruise analysis of all remaining POC samples was conducted at the Marine Science Institute (MSI) Analytical Lab, University of California Santa Barbara, using a CEC 440HA Elemental Analyzer (Control Equipment Corp., now Exeter Analytical) (Hewes et al., 2001; Reynolds et al., 2016; Stramski et al., 1999; Stramski et al., 2008). To minimize uncertainties in POC determinations the measurement protocol included sampling of water between the spigot and the bottommost part of the Niskin bottles to minimize the loss of particles due to settling, low vacuum during filtration (< 125 mmHg) to minimize potential loss of POC due to the impact of pressure differential across the filters, relatively large volumes of sample filtered to maximize the particulate carbon retained on the filter relative to background carbon, proper care during sample handling to reduce exposure to contamination, consistent acidification treatment to remove inorganic carbon from the samples, and determination of final POC values from replicate sample measurements.

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