Dissolved Organic Carbon in the Indian Ocean Dennis A. Hansell University of Miami

Transcription

1 Dissolved Organic Carbon in the Indian Ocean Dennis A. Hansell University of Miami IIOE- 2 Theme 6: Unique Geological, Physical, Biogeochemical and Ecological Features of the Indian Ocean.

2 GLODAP v2 (Key et al. 2015) T, S, O 2, CFCs, TCO 2, TA, pco2, ph, nutrients

3 Survey and Process Studies with DOM Data

4 Monsoon dynamics; hypoxic basin T, S, O 2, CFCs, TCO 2, TA, DOM, nutrients; JGOFS includes many biological variables Agulhas leakage JGOFS IO5 IO6S IO8N IO9N IO8S Ganges-Brahmaputra- Meghna-Irrawaddy Rivers; hypoxic basin Indonesian Throughflow Subtropical gyre; DOC export with OTC

5 JGOFS (1995) Monsoon Dynamics

6 Southwest Monsoon Northeast Monsoon Coastal Upwelling Diatoms & Phaeocystis Convective Overturn Synechococcus

7 Fall Intermonsoon (December) Nitrate (µmol/l) NE Monsoon (Jan/Feb) Spring Intermonsoon (Mar/Apr) SW Monsoon (July/Aug) What is the response in the organic C field?

8 NEM SWM FI C-assimilation (µmol m -2 d -1 ) Chl a (mg m -2 ) SI Observed at mooring (red symbol) Marra and Moore (2009)

9 SI Spring Inter Mid SW TOC stock Late SW SI mswm TOC stock SI Late SWM TOC stock Fall Inter Early NE SI SI NEM Late NE TOC stock FI TOC stock

10 Mean TOC Stock Upper 150 m along the South Line West South TOC (moles/m 2 ) TOC stock varied seasonally by 2 mol C/m2; POC by 0.3 mol/m NE NE SI SI MSW LSW FI NE NE Fractional Date NPP >80% Synechococcus (Liu et al., 1998)

11 TOC STOCKS AND PARTICLE EXPORT Mass Flux (mg/m 2 /d) (Honjo data) J J 400 B 9.5 B 300 J B B 200 Mass Flux B B BB BB B B J B B B B B B B B B 0 B BB Monsoon Status J B B J NE TOC Date SW NE TOC Stock (moles/m 2 ) Large reduction in surface Fe concentration between mid and late SW Monsoon NE Monsoon - convective overturn - low particle export - Large net TOC production SW Monsoon - coastal upwelling - high vertical export - net TOC loss Measures and Vink (1999)

12 Export during the SW Monsoon Diatoms Phaeocystis (Garrison et al., 1998; Latasa and Bidigare, 1998) Arabian Sea Southwest Monsoon: A good location/time for investigating processes controlling export

13 Spring Intermonsoon (following NE Monsoon) SI DOC Stock FI Fall Intermonsoon (following SW Monsoon)

14 Northern Spring Intermonsoon (April 2007) SI Northern Fall Intermonsoon FI Arabian Sea DOC Stock (October 1995) Zonal synchrony, but also along the entire meridian, by coincidence?

15 DOC Export Need to move surface DOC to depth! Low latitude water can export DOC; high latitude waters less so. Modeled DOC Distribution

16 Surface Salinity (July August; WOA) Arrows indicate overturning circulation RSW PGW STMW SAMW SEISAMW AAIW

17 IO8 (80 o E) Salinity Nitrate (umol/l) CFC12 (pmol/l) DOC (umol/l) Ventilation with Subtropical Underwater exports DOC; Ventilation with higher latitude (southern) waters does little to export DOC

18 IO5 AAIW DOC TCO2 STMW SAMW Salinity AAIW What conditions allow DOC to accumulate at the surface, then be consumed at depth?

19 Yukon World s 30 Largest Rivers McKenzie Ob Lena Yenisey Kolyma DOC Flux (TgC/yr) Mississippi Amazon Ganges-Brahmaputra- Meghna-Irrawaddy Rivers Raymond and Spencer 2015

20 Major Rivers into the Bay of Bengal; Associated Salinity 31 Godavari Krisnna Ganges- Brahmaputra - Meghna Mahanadi Irrawaddy Cauvery Bay of Bengal 34.5

21 NE Monsoon ITCZ and Precipitation Eq SW Monsoon L Eq 80% of the annual, continental rainfall occurs during the SW monsoon

22 Bay of Bengal Dynamics DOC (µmol/l) 800 Ganges DOC vs Year Day (1982) River Discharge (m 3 s -1 ) Chl a (mg m -3 ) Ittekkot et al. (1985) July/August Net Community Production (Tg C y -1 ) Sarma 2009

23 Coastal, Fluvial DOC Salinity DOC (µmol/l) Shanumgam et al. 2016

24 I9N/I8S Occupation during Spring Intermonsoon (2007)

25 I9N/I8S Occupation during Spring Intermonsoon

26 P15S Need to distinguish relative and absolute roles of rivers vs precipitation as controls on surface ocean biogeochemistry in this region

27 Salinity J TOC stock J J B 400 B 9.5 B 300 Mass flux J B B 200 B B BB BB B B J B B B B B B B B B 0 B BB B J Questions Posed In an upper ocean system freshened by both major rivers and rainfall, can we distinguish their individual impacts on the biogeochemistry of that system? What are the mechanisms controlling initiation of particle export (and associated scavenging), using the SW monsoon as the model system? What processes allow DOC accumulation at the surface, but then consumption at depth? Who produced that exported DOC? Is it largely of microphytoplankton of upwelling zone, or picophytoplankton elsewhere?