Biogeochemistry of trace elements and isotopes in the Indian Ocean

Sunil Kumar Singh

2 July 2016

Geosciences DivisionPhysical Research LaboratoryAhmedabad 380009

Ministry of Earth SciencesGovernment of India

One of the major discoveries of last decade was the presence of HNLC (High Nutrient Low Chlorophyll) region in world Ocean.

The low productivity was found to be a result of deficiency of key trace elements, e.g.: Fe

Trace elements serve as micronutrients and regulates marine ecosystem dynamics and carbon cycleThey serve as paleo-oceanographic tracers

High Nutrient Low Chlorophyll region of the Arabian Sea

HNLC

Marine ecosystem dynamics and carbon cycling are regulated by trace elements as they serve as micronutrients. For example, Iron is a well known micronutrient, deficiency of which severelyaffects photosynthesis and nitrogen fixation.

The biogeochemical cycling of trace elements, therefore has direct implications to marine carbon cycle, which in turn influences ocean ecosystem dynamics and climate change.

The distribution of TEs in the oceans and their depositional fluxes hold clues to productivity variations / climate change.

Similarly the abundances and isotope composition (e.g., Nd, Pb, U-Th) of water masses are unique and their records in authigenic deposits can serve as paleo-oceanographic tracers.

Trace Elements and Isotopes (TEIs) in the Oceans

GEO-SCIENCES

Important biogeochemical processes in the ocean and the trace metals thought to be fundamental to their action.

DataLimited for Fe

Existing deep ocean Fe data

Stations with Fe concentrations at depths > 2000 m in 2003 (taken from GEOTRACES Science Plan 2006)

Fe data is scarce, particularly in the deep ocean, limited understanding of the Fe cycle

Indian Ocean

ArabianSea

Bay ofBengal

Arabian Sea and Bay of Bengal are unique ocean basinsHigh productivity in the Arabian Sea Arabian Sea is a major denitrification / suboxic ocean basinBay of Bengal is a natural laboratory of river-ocean interactions

(water + particulate)Hydrothermal activityDust InputDistribution of trace elements will be impacted

8

GEOTRACES - India

Technological DevelopmentScience

GEOTRACES - INDIA

ONLY 5-6 Labs worldwide have capability to measure seawater Fe

DFe In SeawaterFlow Injection System for determination of Dissolved Fe at ppt level

SK - 311

SK - 312

OMZ

Remineralisationin OMZ

SK - 304

Sub marine volcanism

Fe input from Kaireiand Edmond vent

fields

SK – 32405-18

SK – 32401-04

SK - 311

Sources of Dissolved Fe in the Indian Ocean and productivity

Fe from Hydrothermal source

Fe from Shelf

Narmada

Tapi

Mandovi

Ganga

Mahanadi

Godavari

Krishna

Bayof

Bengal

ArabianSea

Estuary: Chemical Reactor• REE & Nd: Role of Particulates

0 5 10 15 20 25 30 35Salinity

0

6

12

18

Riv

er-n

orm

aliz

ed N

d

1

4

0.5

Hooghly

Krishna

Godavari

Mahanadi

Dissolved Nd flux : 500 MgDust dissolution : 400 MgModel Estimated Missing Nd: 5500 – 11000 Mg

Nd Flux from particles : 5700±2600 Mg

Nd as tracer of ocean circulation

The Indonesian Throughflow (ITF) and Indian Ocean Dipole (IOD)North eastern Indian acts as conduit between the Indian and the PacificOceanITF transports ~10 million m3/s water from Pacific to Indian OceanHeat Transport of the ITF is 1.09 x1015 WattIt impacts Indian Ocean Dipole, ENSO and hence the monsoonLimited knowledge of transport of water from Pacific to Indian OceanTracking ITF using Nd isotope composition of seawaterWater masses in the Andaman Sea

GEOTRACES - INDIA

Bay of Bengal

Arabian Sea

Eastern Indian Ocean

Indonesian Throghflow

Results indicate that Nd (Nd isotope composition) of water can be used to track Indonesian Throughflow (ITF) as the pacific waters have higher NdImpact of ITF on paleo-monsoon can be assessed using Nd in forams from sediments and corals

Eastern Indian Ocean

fij 1i 1

n

fij xii 1

n

x j

fijNdi Ndjexcess

i 1

n

Ndj

fijNdi NdiNdj

excessNd j

excess

i 1

n

Ndj Ndj

: excess fraction of dissolved Nd in ‘jth’ sample over Ndcontribution from different water masses, respectively.

where,

ijf : the fraction of the ‘ith’ water mass in ‘jth’ sample,

: either potential temperature or salinity of the ‘ith’ water massand ‘jth’ sample, and

excessjNd

ix jx&

Inverse Model Calculation

Excess Nd is derived from sinking detrital material or slope sediments

Excess Nd and their isotope composition

Dust Flux : 7 g m-2 y-1

Excess Nd and their isotope composition

Excess Nd and their isotope composition

Thanks