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by: Aurore MacGyver Sr.


Aurore MacGyver Sr.
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This 8 page Class Notes was uploaded by Aurore MacGyver Sr. on Wednesday September 9, 2015. The Class Notes belongs to OCEAN 600 at University of Washington taught by Staff in Fall. Since its upload, it has received 9 views. For similar materials see /class/192138/ocean-600-university-of-washington in Oceanography at University of Washington.




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Date Created: 09/09/15
OC583 Oxygen Cycle Triple Isotopes Part B I INTRODUCTION 1 A relatively new isotope method has been used to estimate photosynthesis rates in aquatic systems Luz and Barkan 2000 Fig 1 2 Based on an 39 isotopic quot39 of atmosphere 3 Important characteristics a the 170160 of Oz in air is distinctly different from the 170160 of photosynthetically produced 02 4 Therefore measuring the 170160 and 180160 of 0 dissolved in the surface photic layer of the ocean lake river etc yields a direct estimate the proportion of 02 resulting from airwater gas exchange versus photosynthesis 39 oxygen in the A Mass independent fractionation of Oz in the stratosphere Fig 2 Typically kinetic and equilibrium fractionations exhibit a mass dependent effect can be approximated by the following relationship based on a simple harmonic oscillator 3 0cmZml 0cm3m1z where z m3m2m2mlm3ml b for oxygen isotopes this expression yields a value ofz 0529 that would that X17O150 X180150039 2 C for example lfOngoqso 1010 then M70450 100528 ie using 0 notation in 8170150 5 0529 8180150 Ifsl7o150 E 0529 8180150 then a plot of 5170 versus 5180 should yield a slope of 0529 if the simple harmonic oscillator model of vibrational frequencies is accurate However during the formation of ozone in the stratosphere there is a KIE which is mass independent so that the change in the 170160 equals the change in the 180160 a The cause of this mass independent KIE is not clear rotational asymmetriesirlhiemens Science 283 341345 1999 during the subsequent equilibration between oxygen and C02 this mass independent fractionation effect is transferred to C02 which leaves the 170160 of C02 anomalously high and the 170160 of Oz anomalously low in the lower stratosphere N E 4 5 stratospheretroposphere mixing causes some of this isotopically anomalous 02 to enter the troposphere 6 since only a small proportion of the total oxygen in the troposphere contained this mass independent 180160 and 170160 composition the magnitude of this isotopic signature on tropospheric 02 is small B Notation Fig 3 1 Luz and Barkan adopted a new notation to express the triple isotope composition of 02 they used a 17A symbol to express the 17O anomaly which was de ned as follows a 17A 6170 7 0516 51801000 b 17A was expressed using the units ofper meg where 1 per meg 10 1000 N E Luz and Barkan chose AIR as the standard for their 17A scale a thus AIR has a 17A 0 per meg b because AIR has anomalously low 170160 most 17A values will have positive values 4 The coefficient of 05 16 was chosen to represent the slope of 51705180 of 02 produced or consumed during most mass dependently fractionating processes a in particular the ratio of 05 16 was chosen to represent the slope found during the consumption of Oz by respiration i this implies that respiration does not change the 17A of Oz 1 this is important as we will see b Recent measurements of the slope of 51705180 during respiration are closer to 0518 Fig 4 Helman et al Plant Physiology 138 2292 2298 2005 i might need to rede ne 17A to account for recent estimates of slope ii the slope for photorespiration and the Mehler reaction might differ from 0518 but not much data C 17A of Oz in Aquatic Systems 1 As stated the 17A of AIR is 0 per meg because it is the standard 2 The 17A of air dissolved in water is about 16 per meg Fig 5 empirically measured in the lab by Luz and Barkan some recent measurements by other labs indicate 8 18 per meg 3 The measured 170160 to 180160 slope for meteoric water is 0534 not 0516 Fig 6 a this means that the 02 produced during photosynthesis despite being fractionated in a mass dependent manner slope 5 0516 has a 17A that exceeds that of AIR 0 per meg b thus photosynthetically produced 02 has a 17A which depends on the 5180 and 5170 of the water Fig 7 V39 0 for ocean water photosynthetic O has a 17A 249 per meg empirically measured for lakes and rivers the 17A of photosynthetic 0 depends on the 5180 and 5170 ofthe water This means that the 17A of dissolved Oz in the ocean has a possible range of 16 to 249 per meg Fig 8 a If all the O in the surface layer of the ocean was due to gas exchange then 17A 16 per meg b If all the O in the surface layer of the ocean was due to photosynthesis then 17A 249 per meg c If the measured 17A 130 per meg then half the O is from air and half is from photosynthesis Observations of 17A in surface waters Fig 9 a Oligotrophic Ocean BATS and HOTS 3050 pmeg Luz and Barkan Science 2000 Juranek and Quay GBC 2005 b Equatorial Ocean 5090 pmeg Hendricks et al in press Juranek unpub data c Southern Ocean 2050 pmeg Hendricks et al 2004 d Coastal Ocean 30 to 100 pmeg Sarma et al 2005 Munro unpubl data e Sea of Galilee 100 to 140 pmeg Luz and Barkan 2000 Depth profiles of 17A BATS and HOTS Fig 10 17A typically increases with depth underneath the mixed layer b At BATS a clear increase in subsurface 17A during the year Why c Remember Respiration doesn t affect 17A d Depth trends in 17A re ect water mass ventilation history and mixing e In regions of upwelling the mixed layer 17A may re ect both gross photosynthesis rates and the 17A of the upwelled water 11 USING 17A TO ESTIMATE GROSS PHOTOSYNTHESIS RATES A Mass and Isotope Budgets for 02 1802 and 1702 l The surface 02 budget is affected by 02 gas exchange photosynthesis respiration and mixin a dOzdt karnquot solquot pOmm 7 pOZml Photo 7 Respiration i Mixing b d1802dt kam501l18gasp02atmR18atm018sol 7 p02m1R18diss Ph0I0R18Wmer118phmo 7 Respirationquot RlSdiss kdlgresp i Mixingquot R18mix c d1702dt 7 kamsola17gaspOZaLmRl7atm1175017 p02m1Rl7diss Photoquot Rl7Wmer kdl7pm0 7 Respirationquot Rl7dissquot L17resp i Mixingquot Rl7mix E 4 One can combine the 1802 and 1702 budgets using the 17A notation Since we assume that respiration changes the 170160 at a rate 0516180160 then respiration doesn t change the 17A of the dissolved 02 If we ignore mixing we can write two budgets as following Hendricks et al 2004 Fig 11 a dOzdt ka1nsolquot p0mm 7 kamsolp02m1 Photo 7 Respiration b d17AOzdt kam ksol prMm kNAMr 7 kamquot solquot pOZmI AdiSS Photoquot 17Aphm0 7 Respirationquot 17AdiSS assuming respiration doesn t change the 17A of the dissolved Oz c These 02 and 17Aquot Oz budgets can be combined at steadystate to yield i 0 7 kamsolp0mm 17A 717mg Photo17Aphmo 7 Md 7 Resp17Adiss 17Adiss ii Substituting for Resp and rearranging yields PhOtO kalnp2mmSOI39JTUAair 17Adiss17Aphutu 17Adiss d This means that the gross photosynthesis rate in an aquatic system can be estimated by making only one measurement the 1 A of the dissolved Oz and estimating the gas transfer rate kam from wind speed Fig 12 e The ratio of the gross photosynthesis rate gross 02 gas exchange rate equals 17Aair 7 17Adiss17Aphoto 7 17Adiss There are several advantages of using this approach over the traditional 14C uptake method to estimating aquatic photosynthesis or primary production PP rates Fig 13 a In situ PP rates not in vitro PP rates i there are no bottle e cm b Much simpler eld method i No time consuming bottle incubations c Integrates over the lifetime of Oz in the mixed layer i Typically 1020 days ii ie ML depthgas transfer rate eg 50100m 5md d Measures gross PP rates i Not an ambiguous rate between gross and net PP ii Recycling of 14Clabeled OC in the bottle and use of non14C labeled C02 during photosynthesis yield errors in PP rates that are difficult to quantify There are some disadvantages Fig 14 a Measures gross PP rate integrated over the mixed layer depth not the photic layer depth b Uncertainty of method depends on uncertainty of gas exchange rate i30 c Need to convert from 02 production to organic carbon production 1 i39 Typical corrections are 1020 for Mehler reaction which produces 02 but not organic carbon and photorespiration which reduces organic carbon OC production but doesn t change gross 02 production Need to divide 02 production by the Photosynthetic Quotient PQ which can vary from l1 NH4 based PP to 14N03 based PP d In some situations upwelling or mixing can bias the 17A in the mixed layer usually causing an overestimation of gross PP 5 Gross PP rates mg Cmzd estimated using 17A measurements Fig 15 a Oligotrophic Ocean BATS and HOTS 300 to 1500 Luz and Barkan Science 2000 Juranek and Quay GBC 2005 b Equatorial Ocean 10002000 Hendricks et al DSR 2004 Juranek unpub data Southern Ocean 6003000 Hendricks et al in press d Coastal Ocean 100 3000 Sarma et al 2005 Munro unpubl data Sea of Galilee 1600 to 16000 Luz and Barkan f On a global scale ifwe assume an average gross PP rate of 1000 mg Cmzd this is equivalent to 130 Pg Cyr for the magnitude of the biological pump in the ocean about twice current estimates based on 14C PP rates of60 Pg C yr C e 6 Comparison to 14CPP rates Fig 16 a Expected ratio of gross 02 production to 14Cbased organic carbon OC production is about 2 7 27 depending on incubation length 09 3 1 D i Based on comparison of 180 labeled water bottle incubations used to measure gross PP and concurrent 14C bottle incubations to measure PP Marra 2002 If we were to convert the O to OC using a 15 reduction in 02 PP because of Mehler reaction and photorespiration and divide by a PQ 125 half way between 11 and 14 then the ratio of gross PP to 14CPP both in terms of carbon would be 14 18 At BATS and HOTS the observed 170 1313 to 140 1313 rates was 16i04 During CalCOFI cruises where 37 comparisons were made the observed 170 1313 to 140 1313 was 27116 in C units Thus it may be that in situations where PP isn t too variable the 17A method yields PP rates that about 152x those measured by 14C method in agreement with the expected ratio from concurrent 18Olabeled water and 14Clabelled COz bottle incubations In situations where PP is higher and more variable the ratio the 17APP method may yield PP rates that are much higher than 14CPP rates even after correcting the 14CPP rates for the expected offset f There is some evidence that at high PP rates the ratio between gross PP and 14CPP increases Laurie Juranek measured 17APP that was 5x 14CPP during a 10 day storn event at ALOHA Luz and Barkan measured 180PP rates during a spring bloom event in Lake Kinneret that was 8x the 14CPP rate iii However such a trend does not appear in the CalCOFI data to date measured by Dave Munro Fig 16 III USING OzAR 17A MEASUREMENTS TO ESTIMATE NET COMMUNITY PRODUCTION RATES lIncluding a measurement of the 02 saturation state with the 17A measurement provides a means to estimate the net community production rate NCP where NCP gross PP 7 total respiration resulting from phytoplankton zooplankton bacteria etc 2Typically we measure the ratio of the dissolved 02 to dissolved Argon gases as a measure of the biological 02 saturation state the idea is that Ar is an inert analog of 02 the solubility of Ar in water is similar to that of 02 the molecular diffusion rate of Ar is similar to 02 thus the biological 02 saturation state is assumed equal to the measured OzAr relative the OzAr expected if the water was at atmospheric equilibrium at the measured temperature and salinity of the water 4The expression for net community production Photo Resp where Resp is the total respiration rate in the mixed layer is derived from the 02 budget and the expression derived above for gross PP in terms of 17A Fig 17 ie Photo 7 kamp0mn30117Amr 717Adiss17Aphom 717mg the dOzdt kampOZaLmSoll 7 pOzpOzmm Photo 7 Resp assuming NCP Photo 7 Resp and substituting for kam pOgam Sol yields New Photo OzOzatm 1 Apm 17mm Alir 17mm thus the ratio of NCP to gross PP depends on the measured 17A and the degree of biological 02 saturation Oz025m which is equated to the OzArmeas OzAfsafd this ratio is independent of the gas exchange rate to estimate the NCP rate one needs to estimate the gross Photosynthesis rate from the measured 17A and estimated kam 5 Observed NCPgrossPP Figs 18 19 at HOT and BATS 013i003 in the Southern Ocean 017il3 in the Equatorial Paci c 012i0ll in the coastal ocean CalCOFI 016i012 this could be our best estimate of export ratio and ef ciency of the ocean s biological pump 6 Estimated Rates ofNCP mmols Cmzd ie gross PP NCPPPg Fig 20 at HOT and BATS lOiS in the Southern Ocean 13i4 in the Equatorial Paci c 69i62 in the coastal ocean CalCOFI 14i10 globally using an average export rate of 10 mmolsmzd yields 16 Pg Cyr higher than previous estimates of 6 10 Pgyr IVAPPLICATIONS OF 170 131 METHOD Fig 21 1 Improved ability to catch PP events the importance of PP events caused by eddies storms frontal mixing etc is increasing as autonomous monitoring methods improve moorings oats seagliders the likelihood that periodic 624 hour 14CPP incubations would capture these events is small since the 17APP method integrates over 1020 days this method has a much greater chance to represent PP events the inability of 14CPP method to capture PP events has been used as an explanation of why 02 uptake rates typically exceed daytime 02 production rate measured by bottle incubations Karl 1999 2 Obtain large scale surveys of ocean PP rates the relative ease of sample collection means that a large number of PP estimates can be obtained during a typical research cruise collecting four 17A samples per day during a 30 day cruise yields 120 PP estimates even during JGOFS cruises there were usually lt20 stations where 14C incubations were performed using container ships as underway sampling platforms potentially yields repeated basinwide estimates of gross PP Fig 22 3 To better resolve the short spatial and temporal scale variability in marine PP often one sees large variations in plankton abundance as chlorophyll over small spatial scales laments eddies this suggests that there is an associated variability in PP over these small scales yet to date the effort required for 14CPP rate measurements has prevented suf cient measurements to resolve PP rates over small scales the 17A method could be used to resolve short scale PP variations down to about 50km given a 10 day integration time 4Validation of satellite PP rates Fig 22 satellites will be used to determine temporal and spatial changes in PP rates due to both natural variations El Nino NAO PDO and climate change however the satellite based rates of PP need to be validated 17Abased PP rates are well suited for this validation integrate over 10 days which is about the satellite integration time can be obtained over large regions of the ocean in a systematic way eg using container ships V CONCLUSIONS 1 17APP rates measured to date indicate that 14C has signi cantly underestimated gross PP rates in the ocean 2 A consistent ratio of NCP gross PP observed in oligotrophic and coastal ocean suggest that the ocean s ef ciency in recycling 0C is even higher than thought 3 Using 17A and 02 Ar to estimate gross PP and NCP in aquatic systems has the potential to make a major advance in our understanding of the biological pump in the ocean lake rivers etc 4 17APP has the potential to validate satellitebased estimates of PF 5 It will take some time for the oceanographic community to accept the results continued comparisons of 17APP vs 14CPP will be needed


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