Characteristics and sources of the settling particulateorganic matter in the South Adriatic basin

Stefano Miserocchia,*, Jadran Faganelib, Valeria Balbonia, Serge Heussnerc,Andre Monacoc, Philippe Kerhervec

aIstituto per la Geologia Marina, CNR, Via Gobetti 101, 40129, Bologna, ItalybMarine Biological Station, Fornace 65, 66330, Piran, Slovenia

cLaboratoire de Sedimentologie et Geochimie Marines, CNRS, Perpignan, France

Abstract

In the South Adriatic Pit a mooring equipped with two time-series sediment traps and two Aanderaa RCM7current meters was deployed in the frame of EEC-MAST II Mediterranean Targeted Project 1, Subproject

Euromarge-AS. The ®rst trap was located 35 m above the bottom (mab) and the second 500 mab. Sedimentdeposition ¯uxes were recorded biweekly over 18 months (1 April 1994 to 31 October 1995) in the deeper sedimenttrap, and over 12 months (15 November 1994 to 31 October 1995) in the 500 mab trap. The settling material andsur®cial sediment have been analysed for inorganic and organic carbon, d 13C composition, total nitrogen, and

biogenic silica, in order to infer the sources of material.The total mass ¯uxes at both depths were characterized by a high variability and did not present a clear seasonal

trend. Values obtained in the 500 mab trap were generally 2±3 times lower but occasionally similar. The ¯uctuations

were similar in both traps. Based on analyses of inorganic and organic constituents it appears that the fresh organicmatter produced in the surface layer rapidly sank through the water column. Strong seasonal variations of biogenicconstituents occurred. During spring±early summer an input of fresh organic matter prevailed due to the blooms of

mainly siliceous phytoplankton. Late summer and winter samples were characterized by more degraded matteroriginating from calcareous phytoplankton. A signi®cant imprint of terrigenous organic matter was observed insummer and winter 1994, most likely because an unusually high Po River in¯ow was the most important fresh water

source for the Adriatic.A vertical budget of organic carbon has been calculated; only 2.7% of mean annual primary production reaches

the 500 mab trap, suggesting that remineralization processes take place mainly in the upper part of the watercolumn. Comparison between 35 mab trap particle ¯uxes and bottom rain (calculated as sum of remineralization

¯uxes and burial) indicates that a strong near-bottom input of material between the deepest trap and the sur®cialsediment exists due to the lateral advection. # 1999 Elsevier Science Ltd. All rights reserved.

Keywords: Particulate organic matter; d 13C; Carbon export; Biogenic silica; Sediment trap; Adriatic Sea

Organic Geochemistry 30 (1999) 411±421

0146-6380/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.

PII: S0146-6380(99 )00026-1

* Corresponding author. Tel.: +39-051-6398880; fax: +39-

051-6398940.

E-mail address: stefano@igm.bo.cnr.it (S. Miserocchi)

1. Introduction

Particulate organic matter (POM) in the surface

waters on the continental margins mostly originates

from organic matter synthesized by marine organisms,

and from terrigenous sources, mostly by riverine

in¯ows. Organic particles decay and remineralize as

they sink through the water column controlling,

together with water circulation, the balances of oxygen

and nutrients and the distribution of biogenic elements

at the sediment-water interface. The larger, fast sink-

ing, particles are thought to be the principal form by

which the POM can reach sediments with unaltered or

little altered chemical composition (Fowler and

Knauer, 1986).

The Adriatic Sea was selected as an area of study

for the Mediterranean Targeted Project (MTP) (Price

et al., 1997) because of its importance to the mediterra-

nean ecosystem. It represents one of the major shelf

areas of the Mediterranean Sea and major rivers, es-

pecially Po River, contribute a high load of nutrients

allowing eutrophic conditions to develop in its north-

ern part.

In the framework of MTP sediment trap exper-

iments were carried out in the northwestern

Mediterranean, between Marseilles and the Balearic

Islands and in the Kythira Strait west of Creta Island

following the same ®eld strategy and analytical pro-

cedure we used. Average annual ¯uxes were highly

variable ranging from 11 mg mÿ2 dÿ1 measured south

Fig. 1. Map showing mooring site (1030 m depth) in the southern Adriatic Sea

S. Miserocchi et al. / Organic Geochemistry 30 (1999) 411±421412

of Creta to 3788 mg mÿ2 dÿ1 measured on the ¯oor ofLacaze±Duthiers Canyon (northwestern mediterra-

nean). Fluxes were related to the mediterranean cli-mate: higher during late autumn and winter rainyseason, when the river discharge is the highest and

lower during summer, when strati®cation occurs.Particulate ¯uxes were always higher in the near bot-tom trap than in the top trap in all the investigated

sites, indicating the occurrence of lateral advection(Heussner and Monaco, 1996). Measurements of par-ticulate ¯uxes have been performed by means of sedi-

ment traps for a six month period in the Po riverprodelta area (23 m depth). Average ¯ux in the bottomtrap was 6640 mg mÿ2 dÿ1 during December±Marchand 17940 mg mÿ2 dÿ1 in March±April, matching the

Po river discharge (Matteucci and Frascari, 1997).Both experiments suggest a main role for riverineinputs in the control of particulate ¯uxes in the medi-

terranean basin as higher values were recorded inLacaze±Duthiers Canyon and Po river prodelta.Opposite to these sites is the Eastern Mediterranean

that turns out as an oligotrophic, low-sedimentationarea.The main goal of the MAST II-MTP-Euromarge-AS

project was to assess the seasonal and interannualvariability of biogeochemical ¯uxes between riverinein¯ows, the western continental margin and the centraland southern Adriatic basins.

In this context the objectives of the sediment trap-ping experiment were: (1) to measure and analyze the¯ux rates and investigate the sources of settling POM

in the South Adriatic basin using the analyses of or-ganic carbon, its d 13C composition, total nitrogen andbiogenic silica; (2) to evaluate the coupling between

the water column and sediment and; (3) to assess thebudgets of organic carbon, nitrogen and biogenic silicafor this marine basin.

2. Materials and methods

2.1. Sediment trap array

A mooring equipped with two time-series sedimenttraps (0.125 m2 collecting area, 12 receiving cups,PPS3 Technicap) and two Aanderaa RCM7 currentmeters was deployed in the South Adriatic Pit at

1030 m depth (41848.24 ' N, 17819.15 ' E) (Fig. 1). The®rst trap was located 35 m above the bottom (mab)and the second 500 mab. The current meters were

located 2 m below the traps and the sampling rate wasset at 30 min. A scheme of the mooring is representedin Fig. 2. The ¯uxes were recorded biweekly over 18

months (1 April 1994 to 31 October 1995) in the dee-per sediment trap, and over 12 months (15 November1994 to 31 October 1995) in the 500 mab trap.

Fig. 2. Sediment trap array con®guration. The bottom trap

was located 35 m above the bottom (mab) and the midwater

one at 500 mab

S. Miserocchi et al. / Organic Geochemistry 30 (1999) 411±421 413

2.2. Analytical methods

Trap sample tubes were ®lled with 5% formaldehyde

solution bu�ered to pH 7.5±7.8 in GF/F ®ltered sea-

water to prevent organic degradation during trap

deployment. In the laboratory the samples were pro-

cessed according to the procedure described by

Heussner et al. (1990). The main steps of the procedure

were: (1) removal of swimming organisms and; (2) sub-

sampling. The larger organisms were removed with a

wet sieving through 1 mm nylon mesh then the <1 mm

organisms were eliminated by hand under a micro-

scope. The sample was divided into several aliquots

using a precision wet splitter.

Four subsamples were ®ltered through 0.45 mmMillipore ®lters, rinsed with distilled water and dried

at 408C for 24 h for the determination of dry weight.

Total and organic C and total N were determined by

means of CHNS elemental analyser (Fisons NA 2000).

Subsamples for organic carbon were decarbonated

using 1 M HCl and dried (Hedges and Stern, 1984).

Carbonate content was calculated assuming that all

inorganic carbon was represented by CaCO3 and using

a ratio carbonates/carbonate carbon of 8.33. The

reproducibility was satisfactory with an average coe�-

cient of variation for replicate analyses [CV (%)=100

(standard deviation/mean)] of 2.3 and 3.8% for total

and organic carbon respectively. Biogenic silica was

extracted with 0.5 M Na2CO3 solution at 858C for 5 h

(Mortlock and Froelich, 1989), dissolved silica was

determined spectrophotometrically (Strickland and

Parsons, 1972). The analytical reproducibility for bio-

genic silica was within 6% CV.

Analyses of 13C isotopic composition of the CO2,

produced by ignition of organic matter in samples dec-

arbonated with 3 M HCl in an oxygen atmosphere,

were performed with an Europa 20-20 mass spec-

trometer and the results expressed as deviations in -

from the 13C/12C ratio of the Chicago PDB standard

(d 13C). The analytical precision of 13C determinationswas20.2-.Sur®cial sediment samples, down to the depth of

20 cm, were collected by boxcorer and analysed withthe same methodology of trap samples.

3. Results and discussion

3.1. Water masses and currents

Based on temperature and salinity ®elds, at leastthree water masses could be identi®ed in a W±E cross-section of the South Adriatic Pit; namely the Adriatic

Surface Water (ASW), the Levantine IntermediateWater (LIW) and the Adriatic Deep Water (ADW)(Artegiani et al., 1993). The ASW, coming mainlyfrom the northern Adriatic, ¯ows in a thin sur®cial

layer along the western coast from the Adriatic to theIonian Sea. The LIW comprises the layer from 200 to450 m and ¯ows northward in a broad band along the

eastern coast. During summer, it extends westwardsometimes reaching the west coast. The ADW liesfrom 600±1200 m depth, its circulation having a topo-

graphically controlled cyclonic pattern.The temperature and salinity at the 35 mab current

meter ranged from 12.64 to 12.958C, and from 38.43PSS to 38.79 PSS, respectively; at 500 mab they ranged

from 13.10±13.508C, and from 38.46 PSS to 38.88PSS. These T±S values are representative of ADW(Artegiani et al., 1993). The temperature and salinity

values have not shown a seasonal variability.Mean current speeds, calculated for the 2-week

periods of trap sampling, ranged from 1.23 cm sÿ1 to

9.49 cm sÿ1 (average 4.6 cm sÿ1) at the 35 mab currentmeter; and ranged from 1.1 cm sÿ1 to 9.15 cm sÿ1

(average 5.2 cm sÿ1) for the midwater current meter.

Fig. 3. Time series plot of total mass ¯ux at the 35 mab (meter above bottom) trap and at the 500 mab trap in the period from

April 1994 to October 1995

S. Miserocchi et al. / Organic Geochemistry 30 (1999) 411±421414

Absolute speeds were generally su�ciently low (<15±20 cm sÿ1) to consider that ¯ux measurements by the

trap were representative. Current meters depths, calcu-lated by pressure sensor, were quite constant, showingthat the mooring remained in the vertical position

throughout the whole experiment. Current directions,at both depths, con®rmed the general circulation pat-tern in this area: a cyclonic gyre following the iso-

baths.

3.2. Total mass and major constituents ¯uxes of settlingparticles

Total mass ¯uxes were characterized by a high tem-poral variability, varying from 7.7 mg mÿ2 dÿ1 (1±15

August 1995) to 1577 mg mÿ2 dÿ1 (16±31 July 1994)at 35 mab and from 0.9 mg mÿ2 dÿ1 (16±31 December1994) to 453 mg mÿ2 dÿ1 (16±31 March 1995) at 500

mab (Fig. 3). At 35 mab, for which the longest ¯uxrecord was obtained, total mass ¯uxes did not presenta clear seasonal trend: besides the very strong ¯ux

peak of July 1994, secondary peaks were observedduring di�erent seasons.Total mass ¯uxes recorded by the mid-water trap

generally presented the same temporal trend as the

near-bottom ¯uxes. Values were, however, 2±3 times

lower in general, or occasionally similar; the ¯ux washigher only for one sampling period, during the second

half of March 1995, when the 500 mab trap recorded a¯ux of 453 mg mÿ2 dÿ1 and the 35 mab ¯ux was only204 mg mÿ2 dÿ1.The trends of organic carbon and total nitrogen

¯uxes at both depths (Figs. 4 and 5) re¯ected generallythose of total mass ¯ux. Organic carbon ¯ux at the 35mab trap reached its maximum (65.3 mg mÿ2 dÿ1)during the period 15±31 July 1994 with an average of9.27 mg mÿ2 dÿ1. At the 500 mab trap the average or-ganic carbon ¯ux is 8.4 mg mÿ2 dÿ1 with a peak of

24.3 mg mÿ2 dÿ1 during the period 15±31 March 1995.The total nitrogen ¯ux at both depth presented thesame trend of organic carbon with an order of magni-tude lower values.

The biogenic opal ¯ux (Fig. 6) at the 35 mab trapshowed a peak of 163.1 mg mÿ2 dÿ1 during the period1±15 May 1995 and a secondary peak of 96.9 mg mÿ2

dÿ1 during the period 15±31 July 1994. The average¯ux is 25.1 mg mÿ2 dÿ1. At the 500 mab trap the bio-genic opal ¯ux showed a peak of 46.5 mg mÿ2 dÿ1

during the period 15±31 Mar 1995 and an averagevalue of 16.5 mg mÿ2 dÿ1.This wide range of variability in ¯uxes throughout

the sampling period is evidence of an event-dominated

Fig. 4. Time series plot of organic carbon ¯ux in mg mÿ2

dayÿ1 at two depths

Fig. 5. Time series plot of total nitrogen ¯ux in mg mÿ2

dayÿ1 at two depths

Fig. 6. Time series plot of biogenic silica ¯ux in mg mÿ2

dayÿ1 at two depths

Fig. 7. Time series plot of organic carbon content (% dry

weight) of settling particulate matter at two depths

S. Miserocchi et al. / Organic Geochemistry 30 (1999) 411±421 415

pattern that could be better investigated by means ofthe concentrations of the major constituents.

3.3. Concentrations of major constituents

The organic carbon concentrations ranged from 2.5±6.5% at the 35 mab trap and from 3.4±17.8% at the500 mab trap showing a decreasing trend with depth

except for period of 1 15 March 1995 (Fig. 7). At thebottom trap a seasonal trend was not so clear, but awinter period (from 15 November 1994 to 15 April

1995) with very low concentrations was detected.Nitrogen concentrations ranged from 0.3±0.7% at the35 mab and from 0.5±3.7% at 500 mab followingtrends of organic carbon (Fig. 8).

The biogenic silica concentration ranged from 1.9±20.2% at 35 mab trap and presented a seasonal trendshowing 4 peaks in spring periods: 1±15 April 1994

with 17.8%; 15±31 May 1994 with 16.4%; 1±15 March1995 with 20.2% and 1±15 May 1995 with 19.9% (Fig.9). At the 500 mab trap the biogenic silica concen-

tration varied between 6.8 and 17.7%. It was generallyhigher than at the bottom, showing a decreasing trendwith depth with the exception of samples 1±15 and 15±31 March 1995.

The calcium carbonate concentrations ranged from10±26% in the lower trap and from 14±22% in the

upper trap (Fig. 10). Its distribution displayed anopposite trend with respect to biogenic silica: highvalues in summer and winter periods (from 15 June1994 to 15 February 1995 and from 15 August 1995 to

31 October 1995) and an increasing gradient withdepth.A strong seasonal variation of biogenic constituents

occurred in both traps. During spring±early summeran input of biogenic organic matter prevailed due tothe blooms of mainly siliceous phytoplankton. Early

autumn and winter samples were characterized bymore degraded matter originating from calcareousphytoplankton.C/N ratios ranged between 5.3 and 7.9 at the 500

mab trap and between 7.8 and 10.4 at the 35 mab trap(Fig. 11), with the exception of sample 15±31 March1995 with a value of 6.6. At the 500 mab trap the

value was similar to 6.6, the C:N Red®eld atomic ratiofor phytoplankton (Red®eld et al., 1963) suggestingthat the fresh organic matter produced in the surface

layer sedimented rather fast in the water column. The

Fig. 8. Time series plot of total nitrogen content (% dry

weight) of settling particulate matter at two depths. Data in

period from April 1994 to November 1994 are not available

Fig. 9. Time series plot of biogenic silica content (% dry

weight) of settling particulate matter at two depths

Fig. 10. Time series plot of calcium carbonate content (% dry

weight) of settling particulate matter at two depths

Fig. 11. Time series plot of organic carbon/total nitrogen

(atomic ratio) of settling particulate matter at two depths

S. Miserocchi et al. / Organic Geochemistry 30 (1999) 411±421416

ratio increase with depth is probably related to fasterremineralization of nitrogen compared to carbon.

3.4. Sources of the settling organic matter

The d 13C analyses of settling POM revealed 13Cenrichment in June 1994 (ÿ22.1-), January 1995

(ÿ21-) and May 1995 (ÿ21.5-) (Fig. 12). The 13Cdepleted settling POM (<23-) were observed mostlyin the period July±August 1994, and January±

February 1995, about 30±45 days later than the maxi-mal out¯ows of the River Po, which is the most im-portant fresh water in¯ow in the Adriatic area. Theobserved 13C analyses would imply that the settling

POM in the trap most likely re¯ected its marine (auto-chthonous) or terrigenous (allochthonous) provenance.Other interpretations, including temperature induced

di�erences in isotopic fractionation by plankton(Fontugne and Duplessy, 1981) and di�erences inplankton species (Cifuentes et al., 1988; Fry and

Wainright, 1991) seem less probable since analyses of13C of organic matter in sur®cial sediment in the samearea revealed similar values. These values coincide wellwith the west±east, and not north-south, gradient of

di�erences in d 13C values of the Adriatic sedimentaryorganic matter (Faganeli et al., 1994). Moreover, pre-vious analyses of net-zooplankton in the south

Adriatic revealed d 13C values around ÿ20- which fellin the range observed in the northern Adriatic(Faganeli et al., 1988 and 1994). The 13C enriched

settling POM could be, therefore, attributed to theimpact of the sedimented phytoplankton bloomsoccurring about six weeks before. On the other hand,the depleted 13C values would suggest that the settling

POM in that period had a clear allochthonous imprint.Considering the d 13C values of sur®cial sedimentary

organic matter in the same station, averaging ÿ21.9-,

it appears that the observed episodic pulses of settlingterrigenous organic matter were not re¯ected in thesedimentary organic matter.

Fig. 12. Time series plot of d 13C analyses (deviations in - from the 13C/12C ratio of the Chicago PDB standard) of settling par-

ticulate matter at two depths

Fig. 13. Time series of relative contribution of terrigenous (Ft) and marine (Fm) organic matter in the settling particulate matter at

35 mab trap

S. Miserocchi et al. / Organic Geochemistry 30 (1999) 411±421 417

Using the two end-member mixing equation(Fontugne, 1983) for d 13C values of terrigenous (Ct)

and marine (Cm) organic matter in the form:264 d13C � Ftd13Ct� Fmd13Cm

and

Ft� Fm � 1

375we tried to established the percentage of terrigenous

(Ft) and marine (Fm) organic matter in the settlingPOM in the South Adriatic Pit. For d 13Ct and d 13Cm,we used the mean values of terrigenous (ÿ28-) and

marine (ÿ21-) POM from the Gulf of Trieste(Faganeli et al., 1988), and recent data of riverinePOM from the River Po (Marinotti et al., 1997). Thesesomewhat arbitrary chosen end-members, especially of

marine phytoplanktonic POM, merit some comments.It is well known that the d 13C values of phytoplanktonare variable and they re¯ect the water temperature,

nutrient conditions, CO2 ®xation pathways, growthrate, biochemical and species compositions (Altabet,1996). However, previous analyses of the South

Adriatic net-zooplankton (Faganeli et al., 1994)

revealed d 13C values ranging between ÿ19 and ÿ20-which would be about 1±2- higher than the phyto-

planktonic d 13C and in accordance with 13C enrich-

ment in consumers along the food web in the Adriatic

(Malej et al., 1993).

Considering the above described limitations, it

emerges that the settling POM in springs of 1994 and

1995 and early autumn of 1994 was mostly (>2/3) of

phytoplankton origin (Figs. 13 and 14). The corre-

lation between the content of opal and the percentage

of marine organic matter, deduced from d 13C values,

shows a quite good signi®cance level (r=0.968, p <

0.001) (Fig. 15). This correlation suggests that the dia-

toms phytoplankton blooms, occurring normally in

February, May and November in the southern

Adriatic (Pucher-Petkovic and Marasovic, 1982), fall

rather quickly to the seabed. The lower marine percen-

tages of sedimented POM (40±60%) occurred in the

period June±August 1994 and January±February 1995

were most probably due to the in¯ow of the Italian

rivers, especially Po, spreading along the western

Adriatic coast. However, the contribution of the

Albanian rivers could not be omitted because they are

spreading in the north±western direction due to the

gyre present in the southern Adriatic. In the summer

and winter of 1995 this e�ect was not observed

because of the lower, especially River Po, riverine

in¯ows.

During the period 1±15 March 1995 the marine con-

tribution to the POM in the near-bottom sample was

100%, and in the mid-water trap was only 85%, indi-

cating even more biogenic (fresher) material in the

deep trap sample than in the upper one. How can the

deeper sample be fresher than the sample 500 meters

above? Solving this question will help to better under-

stand particle transfer processes. At present we think

that this situation, already described for the northwes-

Fig. 14. Time series of relative contribution of terrigenous (Ft) and marine (Fm) organic matter in the settling particulate matter at

500 mab trap

Fig. 15. Relationship between percentage of marine organic

matter (Fm) (derived from d 13C analyses) and % of biogenic

silica in the trapped particulate matter. The correlation coe�-

cient is r=0.698 and the signi®cance level is p<0.001

S. Miserocchi et al. / Organic Geochemistry 30 (1999) 411±421418

tern Mediterranean (Monaco et al., 1990), results fromdi�erential settling patterns in a context of particletransfer processes dominated by lateral advection.

Both traps could receive material from surface waterlocated upstream of the mooring line, but di�erentialsettling combined with horizontal advection would fuel

the deeper trap with preferentially larger (and/orfresher and/or more biogenic) aggregates.

3.5. Organic carbon vertical budget

A tentative budget for organic carbon at the moor-ing station in the South Adriatic Pit has been made.

Primary production measured during the trap deploy-ment was 316.8 mgC mÿ2 dÿ1 (Malaguti et al., 1997),similar to the value of 317.6 indicated for the Adriatic

Sea in previous work (Dugdale and Wilkerson, 1988).From four measurements performed in the same areaduring 1997±98, the value of 246238 mgC mÿ2 dÿ1

was obtained (Malaguti, personal communication). We

used the overall average value of 262.9244 to calcu-late the budget.Mean particle ¯uxes at 500 and 35 mab have been

calculated during the period when both traps wereworking. The remineralization rates of organic matterreaching the bottom were obtained from benthic res-

piration measurements performed at the mooringstation during the same project from Giordani et al.(1996). The data presented (Table 1) were obtained

from on-deck incubation with chambers carried out in

four replicates to estimate variability. The organic car-bon remineralization ¯ux was a�ected by a high uncer-tainty due to di�culty to reproduce the bottom

condition of such a deep environment during on-deckincubation.The sediment accumulation rate at the trap station

was 0.06 g cmÿ2 yÿ1 calculated from excess 210Pb pro-®le (Frignani et al., 1996). The sediment pro®le of or-ganic carbon down to 18 cm depth (Table 2) showedhigh values on the surface layers and a constant con-

tent of 0.3% below 5 cm depth. The burial rate of or-ganic carbon was calculated assuming that below 5 cmdepth no signi®cant remineralization processes occurs.

The estimate of carbon export was 2.7% for the 500mab trap and 2.4% for the lower trap. These valuesare lower than the values of 8.3% and 4.2%, predicted

for 500 and 1000 m depth respectively from the empiri-cal relationship between productivity at the ocean sur-face and depth-dependent consumption (Suess, 1980).Drifting trap experiments carried out in the Jabuka Pit

(Adriatic Sea), indicated 3.4% as carbon export fromthe photic zone (Miquel, personal communication).Carbon export rate we estimated supports the hypoth-

esis of a higher carbon recycling occurring in the watercolumn compared to oceanic environment.From the sum of remineralization and burial ¯uxes

we can estimate the bottom rain of carbon, nitrogenand silica as high as 89.7, 8.3 and 73.4 mg mÿ2 dÿ1 re-spectively. If these values are correctly estimated, ¯uxes

at 35 mab trap represent only a small fraction of the

Table 1

Budget of organic carbon, nitrogen and biogenic silica ¯uxes in the southern Adriatic basin calculated for the period April 1994 to

October 1995. Downward particulate ¯uxes in the water column were measured by sediment traps. See text for more details. All

¯uxes are given mg mÿ2 dayÿ1. Percentages with respect to primary production are reported in brackets

Organic carbon (mg mÿ2 dayÿ1) Nitrogen (mg mÿ2 dayÿ1) Biogenic silica (mg mÿ2 dayÿ1)

Primary production 262.9

Particulate ¯ux at 500 mab 7.1 (2.7%) 1.2 16.2

Particulate ¯ux at 35 mab 6.2 (2.4%) 0.8 25.8

Remineralization+Burial 89.7 8.3 73.4

Remineralization 84.1260.1 7.1 66.6

Burial 5.6 1.3 6.7

Table 2

Organic carbon, total nitrogen, biogenic silica and d 13C values in a core collected at mooring station in the southern Adriatic Sea

Layer depth (cm) Organic C Total N (% dry weight) Biogenic Si d 13C (-)

0±1 0.536 0.097 0.41 ÿ21.7

1±2 0.466 0.106 ± ÿ21.6

2±3 0.401 0.092 ± ÿ22.0

3±5 0.349 0.074 ± ÿ22.5

8±10 0.322 0.073 ± ÿ21.7

14±18 0.301 0.068 ± ÿ22.0

S. Miserocchi et al. / Organic Geochemistry 30 (1999) 411±421 419

bottom rain (6.9% for carbon, 9.6% for nitrogen and35% for silica) suggesting a strong near-bottom input

of particulate matter.

4. Conclusions

Some interesting conclusions may be drawn from

the particulate ¯ux experiments carried out in theSouth Adriatic Pit using sediment traps.

1. Major constituent composition of trap samplessuggest a seasonality matching the cycle of phyto-plankton production. During spring-early summerthere is an input of `fresh' organic material mostly

composed by siliceous phytoplankton. Late summerand winter are characterized by a more degradedand abundant material originating from calcareous

phytoplankton and terrigenous remains. There isalso evidence that fresh organic matter produced insurface layers can be rapidly exported to depth.

2. The ¯uxes of total particulate matter and of itscomponents increase with depth, suggesting that lat-eral transport of particles originating from the shelfplays an important role feeding the deep basin.

3. The comparison of organic carbon ¯ux with pri-mary production data indicates that a high degreeof organic matter recycling in the water column

occurs, with an export of 2.7% at the midwater trapand 2.4% at the lower trap.

Acknowledgements

This research has been founded in the framework ofthe EEC Mediterranean Targeted ProjectEUROMARGE-AS, contract MAS 2 CT93 0052. We

thank anonymous referees for their useful critical com-ments to the manuscript. Also, we thank R.R. Boniekfor its essential assistance with rope work. This paper

is contribution no. 1156 of Istituto di GeologiaMarina-CNR, Bologna, Italy.

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