Pliocene sequence stratigraphy, climatic trends and sapropel formation in the Northern Apennines (Italy)

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Palaeogeography, Palaeoclimatology, P

Productivity-generated annual laminae in mid-Pliocene sapropels

deposited during precessionally forced periods of warmer

Mediterranean climate

Rossella Capozzi a,b,*, Enrico Dinelli a,b, Alessandra Negri c, Vincenzo Picotti a,b

a Dipartimento di Scienze della Terra e Geologico-Ambientali, University of Bologna, Via Zamboni 67, I-40126 Bologna, Italyb Centro Interdipartimentale di Ricerca per le Scienze Ambientali, University of Bologna, Via S. Alberto 163, I-48100 Ravenna, Italy

c Dipartimento di Scienze del Mare, Polytechnic University of Marche, Via Brecce Bianche, I-60131 Ancona, Italy

Accepted 12 October 2005

Abstract

Paleoproductivity estimates for a sequence of five mid-Pliocene Mediterranean sapropels illustrate the importance of insolation

maxima in enhancing organic carbon accumulation. Well-laminated sapropelitic intervals in the Northern Apennines have been

studied by a combination of sedimentological and micropaleontological analysis, detailed electron microscope description, bulk

chemical composition, carbon-isotopic composition and elemental ratios.

Each sapropel, formed during precessional minima, lasted 7.5 to 10 kyr, which is the same duration calculated for coeval

counterparts in ODP sites in the eastern Mediterranean.

The organic carbon mass accumulation rates of the studied sapropels show the same values of those calculated in the coeval

Mediterranean sapropels, suggesting that the same productivity conditions were controlling sapropel formation in the whole

Mediterranean, despite the differences in depositional setting and the strong variations in sedimentation rate. Mat-forming diatoms

play an important role in increasing the settling velocity, allowing rapid sinking of organic matter and preventing bacterial

remineralization in the water column. The consequent partial oxygen depletion at the seafloor increases preservation, which is

therefore considered an effect, rather than a cause of the organic carbon accumulation.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Sapropel; Annual productivity; Precessional cycles; Mid-Pliocene; Mediterranean climate

1. Introduction

The Pliocene period, from 5.3 to 1.8 Ma, encom-

passes the transition from Earth’s relatively warm cli-

mate to the beginning of Northern Hemisphere

glaciations. The Mediterranean Plio-Pleistocene sedi-

0031-0182/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.palaeo.2005.10.027

* Corresponding author. Dipartimento di Scienze della Terra e Geo-

logico-Ambientali, University of Bologna, Via Zamboni 67, I-40126

Bologna, Italy. Fax: +39 0512094522.

E-mail address: [email protected] (R. Capozzi).

mentary succession often includes bioturbated mud-

stones alternating with laminated layers rich in

organic carbon (sapropels). Hilgen (1991) interprets

these layers as indicators of precession minima and

insolation maxima and he assigns ages to the sapropels

through the correlation and calibration with astronom-

ical cycles. However, despite a clear age assessment,

the origin of sapropels is still a matter of debate (cf.

Emeis et al., 1996; Sancetta, 1999).

Organic-matter accumulation at the seafloor, fre-

quently associated with anoxia, represents an unusual

alaeoecology 235 (2006) 208–222

R. Capozzi et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 208–222 209

depositional setting, given the widespread oxic condi-

tion of the marine bottom waters. This peculiar setting

is usually interpreted as representing primary produc-

tivity peaks and consequent enhanced export of or-

ganic matter and increase of preservation during and

after burial (Meyers and Doose, 1999). Among the

contributions on the long-lasting debate of anoxia vs.

productivity, Pedersen and Calvert (1990) stated:

bhigh primary production and not water column an-

oxia provides the first-order control on the accumula-

tion of organic rich facies in modern oceansQ. Despitesuch a clear point of view, several subsequent papers

invoke both increased primary productivity and pres-

ervation to explain the organic rich facies, as among

the others Nijenhuis and de Lange (2000) for the

eastern Mediterranean Pliocene sapropels. This ambi-

guity is spread in the literature even though several

papers (e.g. Stow et al., 2001) highlight the role of

preservation in enhancing the accumulation of organic

matter in the presence of high sedimentation rates.

Under these latter conditions, dilution of the organic

matter concentration generally occurs (Meyers and

Fig. 1. View of the Fiumana section in the Romagna region of the Northern

sapropel intervals A, C, D and E indicated by arrows. They are characteriz

Doose, 1999). A recent paper by Casford et al.

(2003) documents the role of high export production

to accumulate organic carbon in an anoxic blanket at

the sediment–water interface even in condition of

ventilated water column. To assess the role of these

two mechanisms of organic carbon accumulation (an-

oxia vs. productivity) in different sedimentary envir-

onments, Nijenhuis et al. (2001) compared the

depositional setting of the sapropel-bearing early

Pleistocene Vrica section, exposed on land in southern

Italy, to the coeval eastern Mediterranean sapropels

recovered from ODP Sites. These authors also stressed

the important role of productivity in the formation of

Vrica laminites, where they postulate that anoxia

reached the shallower parts of the eastern Mediterra-

nean basin for only a shorter period relative to whole

of the eastern Mediterranean.

The present study focuses on a section in the North-

ern Apennine (Romagna region) of Italy where mid-

Pliocene laminated horizons occur (the Fiumana section

of Capozzi and Picotti, 2003; Capozzi et al., submitted

for publication). The mid-Pliocene period from 3.3 to

Apennines; black square in the insect indicates the location. Note the

ed by dark colors and by the morphologic relief.

ig. 2. Measured stratigraphic section. Sapropels are labelled from a1

E and correlated to the i-cycles of Lourens et al. (1996). The

ported average sedimentation rate of each individual sapropel has

een calculated by counting the number of couplets per mm in the

amples.

R. Capozzi et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 208–222210

2.9 Ma is particularly suitable to discuss wide climatic

fluctuations related to precessional cycles because the

estimated temperatures were warmer than the warmest

Quaternary interglacials (see review in Leroy et al.,

1999).

The aim of our paper is to assess if processes in-

volved in organic carbon accumulation are the same in

different sedimentary settings and to quantify the con-

tribution of primary productivity in the formation of the

organic-rich intervals.

2. Materials and methods

2.1. The Fiumana section

We chose a section in the Northern Apennines

where marine sediments belonging to the warm Plio-

cene interval crop out (Fig. 1) and can be correlated

with coeval stratigraphic records on land and in the

Mediterranean Sea (see below). In the chosen area, the

Pliocene evolution of the main depositional systems

and their stratigraphic successions have been recon-

structed and biochronologically calibrated (Capozzi et

al., 1992, 1998; Capozzi and Picotti, 2003). Laminated

sapropelitic horizons of middle Pliocene age were

cyclically deposited on slopes and basin plains of the

Northern Apennine foredeep. The Pliocene Fiumana

section, which has been studied in detail for the pur-

poses of the present paper, records the transition from

an early Pliocene perched basin to a middle Pliocene

slope due to the combination of tectonic activity and

infilling of the basin. The organic-rich deposition oc-

curred during a sea-level high-stand of a third-order

cycle (sensu Van Wagoner, 1995) and followed a

climatic optimum for the growth of a nearby carbonate

platform. The depositional environment featured a high

terrigenous supply consisting of prevailing pelites de-

rived from the adjacent uplifting Apennine chain. As a

result, the studied stratigraphic interval includes ex-

panded organic carbon-rich horizons that are in the

range of 2–4 m in thickness (Fig. 1), whereas coeval

pelagic layers in the eastern Mediterranean basin are

some tens of cm in thickness (Nijenhuis and de Lange,

2000). Each individual sapropel has been dated by

correlation with precessional cycles calibrated by Hil-

gen (1991). This sapropel-bearing interval belongs to

middle Pliocene within the Globorotalia bononiensis

zone (Colalongo et al., 1984) and has been early

described as cluster bOQ by Verhallen (1987). The

Fiumana section can be further correlated with the

lower part of the sapropel-bearing section along the

Marecchia Valley, located some 30 km to the Southeast

and described by Rio et al. (1997). Based on these

litho- and biostratigraphic correlations, we were able to

assign each cycle to the insolation i-cycles adopted by

Lourens et al. (1996) and calibrated with the astronom-

ical time scale, and therefore we obtain a numerical

age, starting from i-cycle 294 (sapropel a1, 3.080 Ma)

and ending at the i-cycle 282 (sapropel E, 2.943 Ma)

(Fig. 2).

The stratigraphic section measured and sampled for

this study includes 5 individual finely laminated sapro-

pels with interbedded homogeneous bioturbated mud-

stones (Fig. 1). The sapropel-bearing interval is 52 m

thick (Fig. 2), and is overlain by 20 m of homogeneous

mudstones before the boundary of the overlying lithos-

tratigraphic unit (Lardiano sandstones) consisting of

turbiditic sandstones, whose deposition was forced by

the onset of a low-stand period (Capozzi and Picotti,

F

to

re

b

s

R. Capozzi et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 208–222 211

2003). Table 1 shows in the detail the number and

frequency of samples collected.

The upper four laminated/bioturbated mudstones

cycles, correlating with the cluster bOQ, have been

here named A, C, D and E, label B being omitted to

highlight the missing sapropel that characterizes i-cycle

288. The lowermost sapropel, correlated with i-cycle

294 (Fig. 2), has a different label (a1) because it does

not belong to the cluster bOQ.

Table 1

Sapropels analyzed in the Fiumana section

Depth of

sampling

interval (m)

Samples Optical

microscopy

SEM

(m)

Major, trace

elements,

isotopes,

CHN (m)

Kerog

34.5–34.6 C 9 # 34.545–34.55

34.4–34.5 C 8 # 34.45–34.46

34.1–34.2 C 7 # 34.101 34.10–34.11

33.8–33.9 C 6 # 33.85–33.86

33.5–33.6 C 5 # 33.593–33.60

33.50–33.517

33.4–33.5 C 4 # 33.49–33.50

33.1–33.2 C 3 # 33.14–33.15

32.8–32.9 C 2 # 32.85–32.86

32.5–32.6 C 1 # 32.52 32.50–32.51

29.5–29.6 Biot. 4 # 29.5–2

24.8–24.9 Biot. 3 # 24.80

20.8–21.0 Biot. 2 #

17.7–17.8 A 14 # 17.74–17.755 17.7–1

17.5–17.6 A 13 # 17.55–17.565

17.4–17.5 A 12 # 17.43–17.44 17.4–1

17.1–17.2 A 11 # 17.14–17.15

16.8–16.9 A 10 # 16.83–16.84 16.83–16.84 16.8–1

16.5–16.6 A 9 #16.60

16.545–16.55

16.3–16.4 A 8 #

16.1–16.2 A 7 # 16.10 16.13–16.133 16.1–1

15.8–15.9 A 6 # 15.75–15.765

(Fig. 4)

15.82–15.83

15.5–15.6 A 5 # 15.64 15.55–15.56 15.5–1

15.3–15.4 A 4 #

15.1–15.2 A 3 # 15.12–15.13 15.1–1

14.8–14.9 A 2 # 14.85–14.86

# 14.56–14.57 14.5–1

14.5–14.6 A 1 14.515–14.52

14.50–14.505

4.3–4.5 Biot. 1 # 4.35 4.3–4

Labels A, C, D and E refer to the sapropels intervals of Fig. 2.

Samples labeled as Biot. pertain to bioturbated sediments. The # symbol ind

the optical microscope.

Sapropel intervals have been directly measured on

the outcropping section and 10-cm-thick samples have

been collected every 20 cm in each sapropel. In addi-

tion, we collected 5 samples in the bioturbated intervals

in order to compare the signal to the laminated layers

(Table 1; Fig. 2). Each sample has been oriented and

stored at 4 8C for analytical purposes.

Samples have been analyzed for the whole thickness

with a Wild Heerbrugg optical microscope at 6–50�

en Depth of

sampling

interval (m)

Samples Optical

microscopy

SEM

(m)

Major, trace

elements,

isotopes,

CHN (m)

70.0–70.1 Biot. 5 # 70

51.4–51.5 E 18 # 51.41–51.42

51.1–51.2 E 17 # 51.14.–51.15

51.0–51.1 E 16 # 51.02–51.03

50.7–50.8 E 15 # 50.73–50.74

50.4–50.5 E 14 # 50.47–50.48

50.1–50.2 E 13 # 50.15–50.16

50.0–50.1 E 12 # 50.04–50.05

49.7–49.8 E 11 # 49.78–49.79

49.4–49.5 E 10 # 49.45–49.46

9.6 49.1–49.2 E 9 # 49.19–49.20

49.0–49.1 E 8 # 49.025–49.03

48.7–48.8 E 7 # 48.72–48.722

48.4–48.5 E 6 # 48.43–48.435

7.8 48.1–48.2 E 5 # 48.155–48.17

48.0–48.1 E 4 # 48.05–48.06

7.5 47.8–47.9 E 3 #

47.5–47.6 E 2 # 47.517–47.519

6.947.2–47.3 E 1 #

47.29–47.30

47.28

41.9–42.0 D 16 # 41.91–41.92

6.2 41.6–41.7 D 15 # 41.64–41.65

41.3–41.5 D 14 # 41.33–41.34

41.0–41.2 D 13 # 41.01 41.01–41.02

5.6 40.9–41.0 D 12 # 40.92 40.91–40.92

40.6–40.7 D 11 # 40.641 40.64–40.65

5.2 40.3–40.4 D 10 # 40.35–40.36

40.0–40.1 D 9 # 40.065 40.06–40.07

4.6 39.9–40.0 D 8 # 39.95 39.94–39.95

39.6–39.7 D 7 # 39.64 39.64–39.65

39.3–39.4 D 6 # 39.37–39.38

39.0–39.1 D 5 # 39.02–39.03

.5 38.9–39.0 D 4 # 38.94–38.95

38.6–38.7 D 3 # 38.65–38.66

38.3–38.4 D 2 # 38.35–38.36

38.0–38.1 D 1 # 38.095–38.10

38.005–38.01

icates that the whole thickness of each sample has been observed with

ig. 3. Picture of the interval between 15.58 and 15.88 m above the

ase of the section of the well-laminated sapropel A. The 1.5-cm-thick

ample in the white rectangle is shown in Fig. 4.

R. Capozzi et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 208–222212

magnification, in order to assess the frequency of the

laminae, their composition and grain size. Finally, 19

samples of individual laminae and sets of laminae

(Table 1) were collected within sapropels A, C and D,

and analyzed with a JEOL GSM 2400 (15–200,000�magnification) in order to perform a detailed descriptive

sedimentological and micropaleontological analysis in

each observed lamina.

2.2. Geochemistry

Organic carbon and nitrogen were determined on

duplicate samples using a FISONS NA2000 Elemental

Analyzer (EA) after removal of the carbonate fraction

by dissolution in 1.5 N HCl.

Stable isotopic analyses of nitrogen and organic

carbon were performed using a FINNIGAN Delta

Plus mass spectrometer, which was directly coupled

to the FISONS NA2000 EA by means of a CONFLO

interface. The IAEA standard NBS19 was used as

calibration materials for carbon. Uncertainties are usu-

ally lower than F0.2x, as determined from routine

replicate measurements of a reference sample. Stable

isotopic data are expressed in the conventional delta (y)notation in which the 13C/12C isotopic ratios are

reported relative to the international PDB standard.

Samples for geochemical analyses of major and

trace elements were selected from different levels,

without regular spacing, within the 10-cm-thick sam-

ples of all sapropel layers. Major (Si, Ti, Al, Fe, Mn,

Mg, Ca, Na, K, P) and trace elements (Ni, Co, Cr, V,

Sc, Ga, Cu, Zn, As, Rb, Sr, Y Zr, Nb, Mo, Ba, La, Ce,

Th, Pb, S) were determined by X-ray fluorescence

spectrometry on pressed powered pellets of sediments

homogenized in an agate mortar, using a Philips PW

1480 automated spectrometer following the methods

of Franzini et al. (1972, 1975), Leoni and Saitta

(1976) and Leoni et al. (1982) for matrix corrections.

Long-term reproducibility for major elements was

generally better than 7%, whereas for trace elements

it was on average better than 10%. Absolute accuracy

relative to certified values is generally within the

reproducibility range. Analytical homogeneity between

batches was checked by duplicate analysis of selected

samples and found to be better than 10%.

Mineralogy was investigated on selected samples by

X-ray diffraction (XRD) using a Philips PW 1710

diffractometer using Cu Ka radiation. Analyses were

performed by pressing powder into an aluminum hold-

er, thus obtaining a semiquantitative information on the

main mineralogy. Such a sample preparation does not

enable a detailed distinction among the sheet-silicates.

3. Results and discussion

3.1. Sedimentation rate and duration of sapropel layers

In the Pliocene mudstones of the Romagna Apen-

nine, Capozzi and Picotti (2003) measured an average

sedimentation rate of 0.4 mm year�1. In the mid-Plio-

cene of the Fiumana section, we obtained the same

average value (Fig. 2), which argues for almost constant

terrigenous supply and runoff throughout the whole

sapropel-bearing section. An important feature in this

section is the upward appearing of very thin (max 1 cm)

sand layers, interbedded from the sapropel D upward,

reflecting the depositional history of the area, i.e. the

shelfal progradation during the late high-stand. The

progressive increase of sand is associated to increased

F

b

s

Fig. 4. SEM analysis of individual laminae intervening in the 1.5-cm-thick interval indicated in Fig. 3; arrows indicate the locations of individual

samples. A: SEM micrographs of LL consisting of diatoms (Thalassiotrix–Thalassionema group); B: SEM micrographs of WL consisting of

calcareous nannofossils (Helicosphaera sellii); micrograph B1 shows a WL overlaying a LL; C: SEM micrographs of BL where biofilms have been

preserved at the very base of WL, between coccoliths, and at the base of LL within clays; D: SEM micrographs of BL pyrite framboids, at different

growth stages in relation to the increase of porosity at the base of LL. The pyrite framboids are well developed mainly in pores provided by large

diatom tests.

R. Capozzi et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 208–222 213

thickness of the younger sapropel intervals (D and E)

and to increased terrigenous organic carbon storage.

Laminae 0.3 mm thick occur throughout the lower 2/

3 of each sapropel, whereas in the upper part, laminae

are progressively disrupted by bioturbation, grading

into the overlying bioturbated intervals. Based on opti-

cal and SEM microscopy observations of laminae, we

have identified three main types of laminae: bright

white laminae (WL), less bright light laminae (LL)

and black laminae (BL) (Figs. 3 and 4).

The highest abundance of alternating LL and BL

occur in the middle 2/3 of each sapropel, where WL

occur discontinuously. Below and above this interval we

noticed an increase in the number of WL. SEM analyses

document the occurrence of almost monospecific assem-

blages of pennate diatoms in LL consisting of the Tha-

lassiotrix–Thalassionema group and very rare centric

Hemidiscaceae and Coscinodiscaceae (Bonci, personal

communication 2005) (Fig. 4A). As for the WL these

consist of Helicosphaera sellii monospecific calcareous

nannofossil assemblages (Fig. 4B).

Finally, each biogenic lamina is coupled with a

very thin dark lamina (less than 0.1 mm thick) that

consist of microbial mats and pyrite-rich mudstones

(Fig. 4C and D).

Laminated sediments have been reported in literature

particularly for sapropel S5 (Kemp et al., 1999). Lam-

inae couplets that characterize anoxic recent deposition-

al environments have been interpreted as the product of

annual sedimentation (among the others Wignall, 1994;

Calvert et al., 1991; Negri et al., 2003).WL and LL

consist almost exclusively of well-preserved fossil tests

(calcareous nannofossils or diatoms), that likely reflect

seasonal blooming events. The thickness of each cou-

plet (ranging from 0.25 to 0.5 mm year�1) matches the

average sedimentation rate calculated for the whole

section (0.4 mm year�1), thus we suggest that these

laminae couplets reflect annual deposition.

R. Capozzi et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 208–222214

These considerations allow us to reconstruct in detail

the duration of each sapropel simply by counting the

number of couplets occurring in the studied layers, sub-

tracting from the count the sand layers that clearly reflect

instantaneous events unrelated with sapropel deposition.

The resulting sedimentation rate shows a moderate var-

iability within each sapropel (Fig. 2) and between sapro-

pels and massive beds. Sapropels A, C, D and E lasted

Fig. 5. Profiles of Al2O3, CaO, SiO2 and TOC within each sapropel layer. Ver

bioturbated mudstones; vertical hatched lines represent the average CaO an

from 7.5 to 10 kyr. This duration is very similar to that

calculated for the coeval pelagic eastern Mediterranean

sapropels (Nijenhuis and de Lange, 2000).

3.2. Geochemical characters of the sapropel layers

A general description of the features of the sapropel

layers can be achieved using a combination of bulk

tical black lines represent the average Al2O3 and TOC concentration in

d SiO2 concentration in bioturbated mudstones.

R. Capozzi et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 208–222 215

chemical composition, C-isotopic composition and el-

emental ratios that were used to derive information

about sediment type and environmental conditions at

the time of deposition.

The four sapropels contain a large amount of ter-

rigenous material, as testified by the high Al2O3 con-

centrations (9.30% to 13.72%, Fig. 5), which are

similar to the concentrations in bioturbated mudstones

(12.1 wt.% Al2O3). The highest values occur at the

base and at the top of the sapropel layers, except for

sapropel E where maximum concentrations occur in

the middle.

Fig. 6. Profiles of Zr/Rb, Si/Al, Mo/Al(d 10�4) and Ba/Al(d 10�4) within eac

values in bioturbated mudstones; vertical hatched lines refer to Zr/Rb and M

The concentration of calcium (8.3–20.2 wt.% CaO)

is more variable and is considered to be representative

of the carbonate content. The highest CaO values are

observed in two layers in sapropel E. In general, the

two elements display mirroring trends related to

changes in the ratio between carbonate, mostly biogen-

ic, and terrigenous components (Fig. 5). The carbonate

fraction is mostly biogenic and part of the SiO2 is also

biogenic. Although silica values are not high and their

range is limited (38.8–48.2 wt.% SiO2), there are

marked variations in the Si/Al (Fig. 6), that reflect

changes in amount biogenic Si. These variations

h sapropel layer. Vertical black lines refer to Si/Al and Ba/Al average

o/Al average values in bioturbated mudstones.

R. Capozzi et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 208–222216

could be related to grain size, i.e. the possible presence

of quartz grains, but a geochemical index sensitive to

grain size variations (Zr/Rb; Fig. 6; Dypvik and Harris,

2001; Dinelli et al., submitted for publication) is fairly

constant. SEM observations clearly show the abun-

dance of diatom-rich laminae (Fig. 4), which are

reflected by the Si/Al in sapropels A, D and E.

TOC values in the sapropel layers range between

0.4% and 1.3%, with highest values at the top of

sapropel E (Fig. 5). This latter peak is clearly associated

to the presence of abundant terrigenous carbon debris.

TOC levels are relatively constant in sapropels A and

C, and more irregular in D and E (Fig. 5). Such low

TOC values in the sapropels are comparable to other

on-land Pliocene sapropel layers (see Van Os et al.,

1994; Nijenhuis et al., 2001; Arnaboldi and Meyers,

2003).

Another peculiar feature of these sapropels is the

limited barium content, which, together with the low

TOC, marks a significant difference with open marine

Pliocene sapropels (Wehausen and Brumsack, 1998,

2000; Nijenhuis et al., 2001). In the Northern Apen-

nine sapropels, Ba/Al reaches up to twice the local

background, given by the Ba/Al ratio of the biotur-

bated intervals (Fig. 6). Low Ba enrichments have

been also found in Pliocene sapropel layers of the

Vrica section (Nijenhuis et al., 2001; Arnaboldi and

Meyers, 2003), which were deposited in a water col-

umn not deeper than 1000 m, similarly to our case

study. There are several observations that suggest high

Fig. 7. Profiles of Zn/Al(d 10�4) and V/Al(d 10�4) within each sapropel la

mudstones ; vertical hatched lines indicate V/Al average values in bioturba

productivity (e.g. diatoms and nannoplankton abun-

dance, see below) and actually the low Ba content

might be explained taking into account what suggested

by Von Breymann et al. (1992) that do not record

enrichments in biogenic barium in water shallower

than 1000 m. These conditions of paleobathymetry

and high productivity, which did not permit extensive

accumulation of Ba, however induced the formation of

bottom water anoxia, as testified by the relatively high

concentrations of molybdenum. This element is gen-

erally enriched in sapropel layers and other organic-

matter-rich deposits (Vine and Tourtelot, 1970; Nijen-

huis et al., 1999; Warning and Brumsack, 2000). Mo

is a conservative element in seawater and is concen-

trated in recent sediments deposited under anoxic

conditions (Crusius et al., 1996; Helz et al., 1996;

Dean et al., 1997). Values of Mo/Al are significantly

enriched compared to bioturbated mudstones (Mo/Al

0.40) and show the highest values in the lower part of

sapropel D. Sapropels A and C show intermediate

values of Mo/Al whereas E has the lowest values

(Fig. 6). Other trace metals commonly enriched in

sapropels, such as Zn and V (Warning and Brumsack,

2000; Nijenhuis et al., 2001), are not similarly

enriched (Fig. 7), possibly due to dilution caused by

the high sedimentation rate recorded in the Fiumana

section.

Atomic C/N ranges from 5.8 to 14.8 in the sapropel

layers (Fig. 8), with a value close to 9 in the biotur-

bated mudstones. Scattered low values are associated

yer. Vertical black lines refer to Zn/Al average values in bioturbated

ted mudstones.

Fig. 8. Profiles of y13C(PDB) and atomic C/N in each sapropel layer. Vertical lines refer to average values in bioturbated mudstones.

R. Capozzi et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 208–222 217

with lower values of TOC and Mo, and the more

frequent occurrence of intervening laminae of homo-

geneous grey mudstone suggesting lower productivity

and more oxygenated bottom waters. The C/N ratio

can be used as an indicator for the origin of organic

matter: marine algae have low ratios (4–10) compared

to terrestrial organic matter (Meyers, 1994), whereas

higher values (N16) might indicate organic matter of

terrestrial origin (Calvert, 1983). Variations in the

original signal of marine origin might be related to

preferential removal of nitrogen compared to carbon

during denitrification processes that might affect or-

ganic matter during deposition and are favoured by

suboxic conditions during sapropel deposition (Arna-

boldi and Meyers, 2003).

The y13C isotopic composition of the sapropel layers

ranges between �25.3x and �21.9x (Fig. 8) and do

not differ substantially from bioturbated intervals

(�24.0x on average). Less negative values of y13Coccur in the middle of sapropel A, whereas fairly

constant values are observed in the other sapropels.

The y13C values fall close to the values observed for

marine phytoplankton (�17x to �22x). Lower

values could be related, according to Arnaboldi and

Meyers (2003), to a 13C-depleted inorganic carbon

flux to the algae or to a larger fraction of lighter organic

Fig. 9. Comparison of y13C of organic carbon and atomic C/N ratio of

the Fiumana section sapropel layers. The diagram report also fields

for organic matter derived from marine algae, C3 and C4 land plants

(modified from Meyers, 1994).

R. Capozzi et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 208–222218

matter of terrestrial origin in the sediments. The general

lack of variation in C/N and y13C (Fig. 9) could suggest

no significant changes in the source of organic matter in

the sapropel layers. The increases in the C/N ratio likely

resulted from faster nitrogen recycling in the sedimen-

tary environment during sapropel deposition. On the

other hand, the marine component of the TOC, derived

from kerogen analysis of sapropel A ranges between

47% and 65%, whereas the same component in biotur-

bated intervals is about 10%. This implies that, in this

case, the C/N ratio and y13C cannot help in understand-

ing the origin of the organic carbon.

3.3. Biogenic content

The monospecific calcareous nannofossil assem-

blage consists of H. sellii (Fig. 4B). This is a strong-

ly calcifying species that first appeared at the end of

the Miocene and became extinct in the late Pliocene.

It has been observed forming monospecific layers in

other TOC-rich sediments (Samoggia section, late

Pliocene), and is likely a key species in other Plio-

cene sapropels. It is worth noting that an increase in

the abundance of Helicosphaera spp. in sapropels has

been discussed in several papers (Negri et al.,

1999a,b; Negri and Giunta, 2001). This suggests

that in general the genus Helicosphaera exploited

the paleoceanographic conditions leading to sapropel

formation. Thus, although no literature dealing with

the palaeoecology of H. sellii exists, we can infer that

this species had requirements comparable to the mod-

ern Helicosphaera carteri that shows similar morpho-

logical features. The recent study by Ziveri et al.

(2004) discussed in detail the biogeography of H.

carteri, reporting that it is found in abundance in

the upwelling area off NW Africa. In addition, plank-

ton studies suggest that H. carteri has affinities for

warmer water (McIntyre and Be’, 1967; Brand,

1994). It also shows an affinity for at least moder-

ately elevated nutrient conditions as suggested by

higher abundances in the mesotrophic parts of the

San Pedro basin in the Southern California Border-

land (Ziveri et al., 1995), the Arabian Sea (Andruleit

and Rogalla, 2002) and the Australian sector of the

Southern Ocean. The coccolithophores assemblage of

very oligotrophic (P-limited) western Mediterranean

Sea have very low abundances of this species (Knap-

pertbusch, 1993; Ziveri et al., 2000; Malinverno et

al., 2003). These data suggest that the not regularly

spaced monospecific blooms of H. sellii recorded in

WL are related to high productivity that is mainly

dominated by diatoms as those are more recurrently

observed in the light laminae (Fig. 4A). In fact, the

Thalassiotrix–Thalassionema group occurring in WL is

a typical component of mats produced during high

productivity periods (Bonci, personal communication

2005).

3.4. Glauconite, pyrite and microbial mats

SEM observations of the BL reveal a variety of

components such as peloids of glauconite, pyrite and

microbial mats. bGlauconiteQ peloids are frequent at thebase of sandstones layers with higher permeability, they

are shaped like fecal material or interior of shells indi-

cating their origin. This authigenic mineral grows in

sediments of oxygen-minimum zones that contain high

concentrations of organic carbon and iron in the intersti-

tial waters. Within fine-grained laminae, more reducing

conditions are indicated by the abundance of pyrite

derived from the activity of sulphate-reducing bacteria

whose hydrogen sulphide by-product generates mono-

sulphides and then pyrite. Pyrite framboids are docu-

mented mainly within foraminifera and diatom tests that

provides larger pores for interstitial reactions (Fig. 4D).

Where laminae are impermeable and the biogenic frac-

tion is very small (5–10 Am), the microbial mats have

been preserved (Fig. 4C). We interpret the BL as gener-

ated at the very base of white and light laminae by the

activity of sulphate-reducing bacteria that decomposed

the organic matter formed in the top layers.

3.5. Productivity or preservation?

In order to define the contribution of paleoproduc-

tivity to the sediment record, we focused on the well-

R. Capozzi et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 208–222 219

laminated lower half of sapropel A (i-cycle 290). In this

interval, the percentage of TOC that is of marine origin,

calculated by analysis of the kerogen type, ranges

between 47% and 65%. The TOC of this samples varies

from 0.75% and 0.96%, respectively, and thus the total

percentage of marine organic carbon is in the range of

0.35% and 0.62%.

The contribution of paleoproductivity to the high

sediment accumulation in the described slope environ-

ment has been assessed by measuring the dry bulk

density (DBD), determined by drying ten cubic centi-

meters of sediment at 50 8C on four replicas. DBD is

defined as the weight of dry sediment per unit bulk

volume:

c ¼ Wts=Vt g cm�3��

The calculated mean dry bulk density is 1.5 g cm�3

and provides a measure of compaction. Applying the

relation:

porosity ¼ 100%� bulk density= particle densityð Þ

� 100%

where particle density for most minerals is on average

2.65 g cm�3, the present porosity of the samples is 43%

on average. We assume a saturated condition during the

sedimentation of each lamina with porosity of 70%,

given that wet-porosity of clays ranges between 66%

and 75%, and the latter value pertaining to high organic

matter content. Therefore, the 0.3-mm-thick laminae

originally had to be 0.5 mm in thickness and 0.97 g

cm�3 of original DBD. These values allow us to com-

pare the Fiumana sapropel A to the less compacted

coeval sapropel recovered in the open marine setting

in the eastern Mediterranean and to calculate mass

accumulation rates as:

MAROC ¼ 10� OC� SR� DBD

in which MAROC is the mass accumulation rate of

organic carbon (mg cm�2 kyr�1), OC is the organic

carbon (wt.% of the marine only organic carbon), SR is

the sedimentation rate (cm kyr�1) and DBD is dry bulk

density (g cm�3). Adopting our calculated sedimenta-

tion rates, mass accumulation rate of marine organic

carbon (MAROC) for the sapropel A lies in the range of

170 mg C cm�2 kyr�1 (1.7 g C m�2 year�1) and 300

mg C cm�2 kyr�1 (3 g C m�2 year�1), which is very

similar to that calculated for the same i-cycle in the

open marine ODP 964 (1.2–2.1 g C m�2 year�1,

Nijenhuis and de Lange, 2000).

In the Fiumana bioturbated mudstones, according

to kerogen observations, only 10% of the organic

matter has a marine origin, so the accumulation rate

of marine organic carbon in those sediments drops to

0.15 g C m�2 year�1, which might represent the

primary productivity input during background sedi-

mentary conditions.

Export productivity (EP) can be calculated following

the equation of Sarnthein et al. (1992):

Pexp ¼ 9:354C0:493A S�0:105B�C z0:300

where CA is mass carbon accumulation rate, SB–Csedimentation rate and z water depth.

EP is 13 g C m�2 year�1 for bioturbated mud-

stones and ranges between 43 and 59 g C m�2

year�1 in sapropel layers. All these data are directly

comparable to those reported by Sarnthein et al.

(1992) from direct observation in open marine set-

tings. However, if compared to the amount estimated

by Bethoux and Pierre (1999) of 3 g C m�2 year�1

in the eastern Mediterranean, prior to anthropogenic

eutrophication, the analyzed bioturbated mudstones

show higher export productivity. Moreover, signifi-

cant changes in export productivity have taken

place between periods of deposition of sapropels

and bioturbated mudstones.

Comparison of export productivity with MAROC

allows the assessment of preservation factors in sapro-

pel layers; it ranges between 3% and 5%, showing an

excellent approximation to the highest preservation

factors presently observed (e.g. 5% in the Black Sea,

Arthur et al., 1994). The preservation factor decreases

to 1.13% in the bioturbated intervals, which is a value

very similar to the 1.3% calculated by Bethoux and

Pierre (1999) in spite of a difference of one order of

magnitude in the sedimentation rates.

Proximity to land areas and to sources of nutrients

are key factors for the relatively high export productiv-

ity during deposition of bioturbated mudstones. Since

the sedimentation rates calculated for the bioturbated

intervals and sapropels are the same, this suggests that

runoff was not enhanced during sapropel deposition.

Therefore the nutrient availability in surface water did

not change it is thus likely that primary producers were

favoured by increased solar irradiance that character-

ized insolation maxima periods. During these periods,

phytoplankton blooms (mainly diatoms) formed the

laminae observed in the sapropel. The characteristics

of these laminae, including the original thickness, are

comparable with those that originated by the accumu-

lation of mat-forming diatoms, observed by Kemp et al.

(1999) in the Late Pleistocene eastern Mediterranean

sapropel S5. Therefore this suggests that primary pro-

R. Capozzi et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 208–222220

ductivity and the accumulation of mat-forming diatoms

are the cause of the organic carbon enrichment in the

Fiumana section given that it also shows the same

MAROC of the coeval horizon in the ODP 964 (Nijen-

huis and de Lange, 2000). In addition, data suggest that

the dysoxic–suboxic conditions at the sea-bottom de-

pend on the increase of the EP and on the following

oxygen consumption at the seafloor, similarly to the

model of Casford et al. (2003). In such conditions,

microbial activity at the bottom becomes predominant,

as clearly documented in this paper.

4. Conclusions

The mid-Pliocene Fiumana section in the Northern

Apennine records the deposition of sapropel intervals

interbedded in a sedimentary succession characterized

by high sedimentation rates. Based on our calculations,

sapropels that formed during precession index minima

(i-cycles 292 to 282), were deposited in 7.5 to 10 kyr.

This range is in good agreement with that calculated for

coeval sapropels of the eastern Mediterranean by Nijen-

huis and de Lange (2000).

About two-thirds of each sapropel consists of lam-

inated sediment formed by couplets of laminae made

up of tests of diatoms and/or coccolith alternating with

organic-rich and pyrite laminae. Each couplet is the

record of annual deposition under high primary pro-

ductivity regimes. The mass accumulation rates of the

organic carbon for the sapropel intervals of the Fiu-

mana section have the same values of those calculated

in the coeval Mediterranean precession-related sapro-

pels (Nijenhuis and de Lange, 2000). Our estimation

of the export production is four times that calculated

in bioturbated mudstones, which, in turn, are high

compared to open marine settings. Nutrient content

in the surface water might be assumed constant and,

in this condition, the hypothesis that increased supply

of nutrients from land controls productivity during

sapropel formation (e.g. Rossignol-Strick, 1985;

Meyers and Doose, 1999) weakens. Primary produc-

tion is likely favoured by enhanced solar radiation

during periods of maximum solar insolation, and

not by higher nutrient input due increased runoff.

These factors have a basin-scale effectiveness in the

whole Mediterranean, despite the different deposition-

al settings.

This paper documents the role of mat-forming dia-

toms as an important cause also for increasing the

settling velocity and the rapid sinking of organic matter,

preventing bacterial remineralization in the water col-

umn and its accumulation at the bottom (Kemp et al.,

1999; Bianchi et al., 2005). The remineralization causes

oxygen depletion at the seafloor, but not always devel-

oping anoxia. Therefore, the increased preservation of

organic matter is an effect of the high productivity

rather than of anoxia.

Acknowledgments

M. B. Cita and W. Dean are warmly thanked for

suggestions and comments that improved the earlier

version of the manuscript. Funds by RFO (60%) are

gratefully acknowledged. The authors are indebted with

Paolo Ferrieri for assistance during SEM investigation

and micrographs acquisition.

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