Indian Journal of Marine Sciences

Vol. 36(4), December 2007, pp. 399-406

Organic matter distribution pattern in Arabian Sea: Palynofacies analysis from the

surface sediments off Karwar coast (west coast of India)

Vandana Prasad*, Rahul Garg, Vartika Singh & Biswajit Thakur

Birbal Sahni Institute of Palaeobotany, 53, University Road, Lucknow-226 007, India

[*E-mail- vanprasad@yahoo.co.uk ]

Central Arabian Sea region, situated off the Karwar coast, is characterized by intense mid depth (~120-1200 m) oxygen

minima zone and shows preservation and accumulation of relatively high organic matter content. Palynofacies analysis was

carried out with a view to understand the organic matter production, preservation and degradation in the surface sediments

of Arabian Sea from 15 m-2750 m depth off Karwar [14.47-14.40° N -70.77-74.25°

E transect]. Palynofacies analysis, which

involves qualitative and quantitative estimation of terrestrial and marine organic matter is a useful tool to decipher and

assess paleoenvironmental changes in various water depth zones from shelf-slope region off the Karwar coast. There is a

marked change in the palynofacies characteristics of the organic matter recovered from various depth zones. High SW

monsoonal activity over Karwar coast results in increased runoff and nutrient loading in the coastal waters. This enhances

primary productivity in the inner shelf region. Organic walled dinoflagellate cysts as important constituent of primary

productivity, predominate under such conditions, being converted into Amorphous organic matter (AOM) as a result of

degradation under low oxygen environment. Hence, in the present study high AOM is used as proxy for the low oxygen

content at sediment-water interface. It also provides evidence of high primary productivity in the photic zone. Study further

reveals that terrestrially derived charcoal and woody plant tissue resistant to degradation, are transported to continental slope

regions at greater depths. Occurrence of a large proportion of well preserved labile organic matter (exoskeleton fragments of

planktonic crustaceans) and AOM in the mid-outer slope surface sediments indicate enhanced primary productivity and high

rate of burial efficiency making these areas, characteristic of low oxygen. The study shows that the Karwar coast margin is

highly productive as a result of runoff related nutrient loading and is the primary cause for oxygen minima conditions.

[Key words: Palynofacies, primary productivity, oxygen minima zone, Karwar coast, Arabian Sea, west coast of India,

organic matter, sediments, amorphous organic matter]

Introduction

Arabian Sea is amongst the most productive

regions in the world ocean and is characterized by one

of the largest bodies of oxygen deficient waters on the

Earth1. A stable oxygen minimum zone (OMZ) exists

in the northeastern Arabian Sea characterised by

greater accumulation of organic matter2, 3

. Since

organic matter is supplied to marine system from both

terrestrial and marine sources, it becomes essential to

characterize various organic matter types to decipher

runoff and productivity related changes. Identification

of modern analogues for the environment of

hydrocarbon source rock formation requires

compilation of information on the distribution of

organic carbon in marine bottom sediments. More

recently, there has been considerable interest in

carbon cycle in the ocean, which involves settling and

burial fluxes of carbon and associated biogenic

elements at discrete oceanic sites. During past few

decades, palynofacies studies are being used in the

hydrocarbon exploration as well as in deciphering

paleoenvironment of pre-Quaternary sediments4-7

.

Palynofacies study has an advantage over bulk

geochemistry as it provides direct means to identify

source and constituents of particulate organic matter

assemblages and hence has been effectively used in

paleocenographic interpretations7-10

. Vast amount of

data have been generated on the geochemical aspects

of organic matter characteristics in marine sediments,

however, the mechanisms controlling the correlation

between organic matter production, composition,

selective preservation and sedimentation in Arabian

Sea sediments are still insufficiently understood.

Wiseman & Bennette11

first studied the distribution of

organic matter in the sediments of Arabian Sea. Murty

et al.12

gave an account of the depth wise distribution

of organic matter in surface samples collected from

the shelf and slope regions off Bombay, Karwar,

_________

*For correspondence

Phone: +91-0522-0983922542

Fax: +91-0522-2740485

INDIAN J. MAR. SCI., VOL. 36, NO. 4, DECEMBER 2007

400

Mangalore, Cochin and Alleppey. Their study showed

that the sediments from inner shelf and slope regions

have high organic content whereas the sediments of

outer shelf are comparatively poor in organic

content12

. These studies mainly deal with the

geochemical aspects of organic matter and do not

provide any information as regards to the nature of

organic matter production, its provenance, morpho-

logic characteri-zation and selective preservation in

the Arabian Sea sediments.

Palynofacies studies that involve relative distri-

bution pattern of various organic matter types in the

marine sediments can be effectively used as an aid in

paleoenvironment and hydrographic interpretations7, 10

.

Complementary to geochemistry, palynofacies studies

can be a useful tool in deciphering variability in the

primary productivity, differential transport of organic

matter and its preservational potential during

sedimentation as well as its differential degradation7.

Its advantage over other techniques is that it provides

a visual assessment of the origin, characteristics,

provenance and state of preservation of organic

matter7. It has been demonstrated that the degree of

organic matter preservation are positively correlated

with sedimentation rate and that the nature of

sediment affects the rate and extent of organic matter

degradation3. Both terrestrial and marine organic

matter biomacromolecules are of different origin and

hence show different nature and resistance to

degradation7.

Continental shelf break in Arabian Sea13

varies

considerably ranging from 132-140 m off Bombay

and 100-115 m off Ratanagiri and continental slope

from 200-2600 m. Shelf break is gradual in the area

off Bombay but becomes steeper towards southern

region13

. The sediment varies from silty sand to sand

in the inner to outer shelf region, however, slope

region shows progressive fining of sediment from

outer-shelf-slope region13

.

The present study deals with the documentation,

distribution and selective preservation of various

sedimentary organic matter types in the surface

sediments of Arabian Sea from continental shelf-slope

[from 15 m-2750 m depth in the 14.47-14.40° N

-

70.77-74.25°-E transect] off Karwar coast, and to

evaluate the relative proportion of phytoplanktonic

and zooplanktonic remains and relate them to their

productivity and preservational potential according to

changes in the water depth.

Materials and Methods Nine surface sample (SC 21 - SC 28) covering both

shallow shelf to slope region (15 m-2750 m) at

latitude 14.47-14.40° N and longitude 70.77-74.25° E

collected during cruise of ORV Sagar Kanya

(SK-117), October 1999, from the region off central

west coast of India (Fig. 1). The samples were

provided by NIO, Goa for the study of palynofacies

analysis. Samples were dried and 2 gm of each were

used for palynological processing. The carbonate was

dissolved with HCl (10%) and silicates with HF

(40%). Use of oxidizing reagents (HNO3) was avoided

in order to analyse organic matter as a whole and to

avoid alteration of organic matter preservation during

chemical processing. Slides were made with polyvinyl

alcohol and mounted in Canada Balsam.

Thirteen sedimentary organic matter (SOM)

categories were chosen to represent the total SOM

assemblages. Their biological sources and

constituent’s sensu Tyson7 are summarized. In order

to quantify the individual components as percentages

of the SOM, a minimum of 200 counts were made for

each sample.

Distribution patterns of palynodebris or

sedimentary organic matter have been extensively

used for environmental interpretation of Pre-

Quaternary sedimentary successions8, 14

. von Waveron

& Visscher15

applied palynofacies analysis

interpretation in modern sediments for environmental

and burial efficiency of organic matter. In the present

study, individual palynodebris components were

classified into the categories, based on the physical

characteristics of recovered organic matter (Table 1).

Fig. 1—Location of surface samples collected at various depth in

Arabian sea off Karwar coast.

PRASAD et al.: PALYNOFACIES OF KARWAR COAST

401

Results and Discussion In the present study, terrestrial debris (degraded

brown, structured) and marine debris (amorphous)

dominated in the inner shelf sediments (Fig. 2). Shelf

region 15 m-147 m depth shows enhanced peridinioid

dinocysts and decrease in black oxidized debris and

gonayaulacoid dinocyst content. Low salinity tolerant

dinocyst taxa Tuberculodinium vancompoae occurs in

inner shelf sediments and shows sharp reduction in

the outer-shelf region (Fig. 2). Gonayaulacoid

dinocysts along with amorphous organic matter

dominate in the inner-mid slope sediments. However,

the mid-outer slope sediments are rich in black and

brown degraded debris, labile organic matter,

copepod egg envelopes, scolecodonts, tintinnids and

various other zooplanktonic organic matter remains

(Figs 2 and 3). The inner slope sediments around

200 m depth are also rich in organic matter content,

albeit dominated by Cyanobacterial remains

(Figs 2 and 4).

The distribution of allochthonous fraction (black,

brown degraded and structured debris) in the marine

environment is controlled mainly by physical

processes. By contrast, the autochthonous fraction

(dinocyst, tintinnids, scolecodonts, copepod egg

envelopes and amorphous organic matter) can be

significantly affected by biological/ecological

processes10

.

The hydrodynamic conditions together with relative

buoyancy of organic particles seem to be the essential

factor controlling the distribution of the allochthonous

organic fraction in marine environment10,

15

. In

addition to that, the degree of resistance to decay also

plays an important role in the distribution of organic

particles in the marine sediment7, 15

. Black oxidized

debris are opaque fragments commonly designated as

inertinite and originate from highly oxidized higher

plant tissues. Usually occurring as blade shape

particles, these are highly buoyant, hence their settling

to the sea floor is delayed as in the case of the mica

flakes14

. Selective degradation of organic matter also

plays a very important role at the time of burial in

marine environment. The aromatic compound derived

from lignin, are broken down very slowly, which

shows that the lignin rich woody debris occuring in

greater portion in black and brown debris have greater

chance of survival in marine sediments7, 10, 14

. These

are characteristically found in abundance in the outer

slope sediment in the present study. Amorphous

organic matter, mostly derived from decomposition of

phytoplankton remains, tends to form fluffy

aggregates known as “marine snow”7. Being highly

susceptible to oxidation, their presence in the marine

sediments indicates reducing conditions in the water

column and sediment water interface7. Hence, large

proportion of amorphous organic matter content in the

slope sediments further points to the prevalence of

low oxygen conditions in this region. Amongst the

zooplankton remains, the transparent, subangular,

pentagonal-hexagonal shaped particles identified as

exoskeleton fragments of planktonic crustaceans15

,

represent the most labile (low resistant) fraction of the

zooplankton remains. In the present study dominance

of pentagonal-hexagonal exoskeleton fragments of

planktonic crustaceans in the mid-outer slope

(1900 m-2750 m) region provides evidence of low

oxygen environment or extension of OMZ in the

slope region. It is interpreted that the organic

aggregates or the “marine snow” produced during

the period of phytoplankton blooms, tend to show a

Table 1—Sedimentary organic matter types documented in the present study (Modified after Tyson7, and

von Waveren & Visscher15)

Group Category Characteristic features and Origin

Black oxidized (charcoal) Oxidized land plant tissue

Degraded brown Partially degraded land plant tissue

Structured Well preserved land plant tissue

Palynodebris

Amorphous Unstructured homogenous material of marine origin

Peridinioid dinocyst Primary consumers, marine origin

Gonyaulacoid dinocyst Primary producers, marine origin

Tuberculodinium vancampoae dinocyst Primary producers, marine origin

Palynomorphs

Cyanobacteria Primary producers, Fresh water and marine origin

Tintinnids (loricae or cyst of tintinnids) secondary consumers, marine origin

Scolecodonts Mouth parts of predominantly planktonic annelids, marine origin

Copepods Egg envelopes of copepod egg, marine origin

Zooplanktonic

remains

Labile organic matter Transparent exoskeleton fragments (various shapes) of planktonic

crustaceans, marine origin

INDIAN J. MAR. SCI., VOL. 36, NO. 4, DECEMBER 2007

402

Fig. 2—Distribution pattern of sedimentary organic matter types in various water depths.

PRASAD et al.: PALYNOFACIES OF KARWAR COAST

403

Fig. 3—Palynomorphs and zooplanktonic remains in palynological assemblages from surficial sediments of the Karwar coast, Arabian

Sea. Scolecodont sp. 1327273, 32Q1, Scolecodont sp. 1327274, 21Q4, Foraminiferal test linning 1327280, 27P1, Tintinnid 1327274,

33G4, Copepod egg envelope 1327275, 20M4, Copepod egg envelope 1327274, 24H2, Copepod egg envelope 1327274, 13F4,

Microthyraceous fungi 1327274, 20U2, Protoperidinioid dinocyst (Lejeunecysta sp.) 1327276, 16J1, Protoperidinioid dinocyst

(Votadinium calvum) 1327273, 16H3, Protoperidinioid dinocyst (Trinovantedinium sp.) 1327273, 20H4, Protoperidinioid dinocyst

(Quinquecuspis sp.) 1327279, 48N1, Protoperidinioid dinocyst (Stelladinium sp.) 1327277, 23Q3, Protoperidinioid dinocyst (Round

brown) 1327272, 32G2, Gonyaulacoid dinocyst (Spiniferites sp.) 1327276, 24E1

INDIAN J. MAR. SCI., VOL. 36, NO. 4, DECEMBER 2007

404

rapid downward flux and in the process other organic

particles and mineral matter get attached to the

aggregates, increasing the burial efficiency. The high

organic matter burial fluxes will have the effect of

protecting more labile organic material from

microbial attack. Organic particles with less residence

time in the water column would result in lesser period

of degradation from aerobic bacteria. Hence, relati-

vely high amount of most labile crustacean remains in

the mid-outer slope sediment not only indicates high

productivity zone but also high burial efficiency.

It is considered that where the oxygen minimum

layer intercepts the continental slope, the bottom

water and some times the sediment water interface

becomes anoxic16, 17

. The intense oxygen minimum in

the Arabian Sea where it intersects the western Indian

continental slope, shows zone of high organic carbon

values with concentration reaching 6.9 wt % on the

inner-mid slope18, 19

between 200 m-1500 m. Thiede

& van Andel20

and Slater & Kroopnick21

stated that

the accumulation of organic matter content in the

bottom sediment is controlled by the oxygen content

Fig. 4—Sedimentary organic debris in palynological assemblages from surficial sediments of the Karwar coast, Arabian Sea. Black

oxidized debris (charcoal) 1327276, 30Q3, Brown degraded debris 1327277, 21O4, Brown degraded debris 1327278, 12Q2, Structured

Debris 1327272, 33F2, Brown degraded debris 1327273, 24N1, Amorphous organic matter along with copepod egg envelope 1327278,

14E2, Amorphous organic matter originating from decaying dinocyst 1327278, 19M2, Decaying Spiniferites Sp dinocyst 1327278, 23T2,

Amorphous organic matter originating from decaying dinocyst 1327278, 23U3, Cyanobacteria (Microcystis sp.) 1327280, 31O1,

Transparent exoskeleton fragments of planktonic crustacean (labile organic matter) 1327276, 32N4, Transparent exoskeleton of

planktonic crustacean (labile organic matter) 1327274, 16D2

PRASAD et al.: PALYNOFACIES OF KARWAR COAST

405

of the overlying water. Whelan & Farrington22

,

considered that the organic matter content of the

sediment is strongly and positively correlated with the

silt and clay content and shows that the sediments

occurring in outer shelf region are generally coarse

grained with high CaCO3 and low carbon values.

However, the amount of sand decreases seaward, so

that the inner slope sediments are significantly fine

grained. It is suggested that the lateral transport of

organic matter plays an important role in the

distribution, quality and burial of organic matter along

continental slopes in high productive areas23

. In the

present study occurrence of well preserved highly

resistant black oxidized terrestrial plant debris in mid-

outer slope region is quite interesting. Even if such a

long distance transport is evoked, their highly buoyant

nature rules out downward settling to such great

depths in any reasonable period of time. Alternatively,

it may probably suggests spread of Indus Fan during

LGM when the sea level was considerably lower

(~120 m) than the present to facilitate their quick

transport and burial. However, it remains highly

speculative and needs to be backed by C14/AMS

dates to reach to any logical conclusion.

It is surmised that the marginal areas or inner shelf

region of eastern Arabian Sea are generally much

more productive in terms of primary productivity.

High SW monsoonal activity in Karwar region24

leads

to high runoff in Arabian Sea. High nutrient loading

induces primary productivity in the euphotic zone that

increases the organic production in the upper water

column which ultimately leads to degradation beneath

the euphotic zone. The oxygen consumption increases

with depth under areas of high primary productivity,

mainly due to microbial respiration during the

degradation of organic matter16

. Hence, oxygen

minimum layer is formed beneath the euphotic zone

where oxygen demand is greater than supply and is

also maximum in carbon dioxide and nutrient

content16

. Decomposition of large amount of

phytoplankton leads to high amorphous organic

matter production below the euphotic zone within

oxygen minima layer7. Moreover, the low oxygen

concentration in the water column also prevents its

degradation and helps in preservation in the

sediment7.

The intensity of oxygen depletion is dependant on

the residence time of oxygen minima layer within the

water column and the productivity of the overlying

euphotic zone12

. Hence, occurrence of large amount

of organic detritus including amorphous organic

matter in the eastern Arabian Sea sediment all along

the inner shelf to mid slope region provides evidence

for considerably stable oxygen minima conditions in

this region.

Acknowledgement

The authors express their thanks to Dr N C

Mehrotra, Director BSIP and Dr S R Sheyete

Director, NIO, Goa for encouraging the present study

under a collaborative programme. They are also

grateful to Dr R Nigam, NIO, Goa for providing the

samples under the MOU between NIO and BSIP and

help during the course of investigations.

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