The Holocene sedimentary regime in the northern Mid-Atlantic Ridge region

Earth and Planetary Science Letters, 78 (1986) 271-287 Elsevier Science Publishers B.V., A m s t e r d a m - Printed in The Netherlands

271

[6]

The Holocene sedimentary regime in the northern Mid-Atlantic Ridge region F.E. G r o u s s e t 1,, a n d R. Chesselet 2 ; Centre de Recherche sur rEnvironnement Sbdirnentaire et Structural des Domaines Marins, U.A. C N R S 197, Universit~ Bordeaux I, 351 Crs. de la Libkration, 33405 Talence Cedex, (France) 2 Centre des Faibles Radioactivitks, Laboratoire mixte CNRS-CEA, Domaine du CNRS, 91190 Gifsur Yvette (France)

Received August 6, 1985; revised version received March 3, 1986 The Holocene (10,000 yr B.P.) Mid-Atlantic Ridge sedimentary regime between 4 5 ° N and 6 5 ° N has been investigated by examination of sediments from 70 cores. Smectite, chlorite and quartz, 87Sr/86Sr isotopes and REE have been used as conservative tracers to provide source identification and to calculate accumulation rates. The entire Holocene sedimentary regime in this part of the Atlantic can be attributed to particle fluxes from two sources: the Icelandic and North American systems. Two main transport mechanisms are identified: (1) Particles derived from the Icelandic system have been tranSported southwards, as far as 4 5 ° N , by bottom currents. (2) Material coming from North America is mainly transported by the aeolian regime. At 4 5 ° N , the windborn accumulation rate is estimated to be 0.5 g c m 2 kyr l, which is identical to the accumulation rate of bottom-advected material from Iceland. Finally, a diagnostic model for the North Atlantic sedimentary regime is proposed.

1. Introduction

In the North Atlantic Ocean, the non-carbonate fraction of the sediments is generally considered to consist of detrital, continental-derived products. The contribution of the Mid-Atlantic Ridge (MAR) submarine volcanic processes to the oceanic sediments is not well known. Are there important hydrothermal authigenesis as previously observed in the Pacific Ocean? What is the contribution of the emergent volcanic areas: Iceland, the Faeroe, the Azores? What is the contribution of the continents themselves? As discussed by Turekian [1], Biscaye [2] and Goldberg [3], several relationships have been identified between the large sedimentary provinces of the North Atlantic and a variety of continental source regions. Recent studies, however, have emphasized that the M A R system acts as a source of sedimentary particles [4-6]. These studies used essentially qualitative information, so we do not know the partitioning between the continental and the "crustal" contributions. Only Turekian [1] * Present address: Lamont-Doherty Geological Observatory of Columbia University, Palisades, NY 10964, U.S.A. 0012-821X/86/$03.50

© 1986 Elsevier Science Publishers B.V.

estimated the accumulation rates of clay minerals ( < 2 /~m) in the North Atlantic. His estimate was 1.1 g cm 2 kyr-1, and suggested that a significant contribution is derived from the North American continent. Besides remaining uncertainty in sediment sources, the transport mechanisms of these particles are not well known: Biscaye [2] invoked turbidity currents and other down-slope processes as well as transport by bottom currents; Goldberg [3] and Windom [7] suggested a high wind transport contribution; finally, concerning the Icelandic detrital products, Grousset [8] proposed a deep bottom current transport. Our study was undertaken to differentiate the types of fluxes important to the North Atlantic sedimentary regime, to identify their origins and to propose transport mechanisms which explain the observed distribution patterns. One of the main problems was to evaluate the input of volcanogenic particles from the M A R and to compare this input with that of allochthonous materials. In order to deconvolve the various sources within the sediments, the various source signals, we have used a geochemical approach similar to that which was successfully used in the Pacific

272

R / V "Jean Charcot" to the Icelandic basin and the Reykjanes Ridge in July 1977 (Faegas I) and of the R / V " L e Noroit" along the axis of the MAR from the Reykjanes Ridge to the Azores in July 1979 (Faegas II) [8]. These samples were recovered by raising box, piston and gravity cores at each station. For most of the stations, living plankton samples were also collected, filtered, fixed and analyzed using the methods and collaboration of Pujol [10].

Ocean by Heath and Dymond [9]. We studied a number of tracers of provenance: minerals (smecrite, chlorite, quartz), trace elements (Ta, Sc, Rb, Th, REE) and isotopic ratios (S7Sr/S6Sr). We selected only "conservative tracers", i.e. minerals, elements or parameters for which the fluctuations in concentration are due only to physical dilution and not by chemical processes such as dissolution a n d / o r precipitation. The observation of conservative tracer patterns allow us to identify gradients related to specific sources or end-members, which then may be explained in terms of transport mechanisms. Finally, by the use of mixing ratios between the end-members, we propose calculations of the contribution of each source to the sediments of the MAR between the Azores and Iceland. This study concerns sediments of the Holocene period (10,000 yr B.P. to today), the limits of which were recognized by foraminifera analyses [10], isotopic stratigraphy [11] and tephrochronology [12].

2.2. Analytical procedures The carbonate content (wt. % CaCO3), consisting essentially of planktonic foraminifera and pteropods was measured gasometrically (error < 5%) and is used to express our data on a carbonate-free basis (CFB). Porosity values were obtained by measuring the interstitial water content gravimetrically before and after drying at 60°C in an oven, in order to express mass, rather than linear accumulation rates (error < 5%). For each sample, the grain-size distribution was determined by X-ray absorption analysis (Sedigraph Coultronics 5000 S), after HzO2 and ultrasonic treatments, These results give the size distri-

2. M e t h o d o l o g y

2.1. Sampling Most of the samples used in this study (Fig. 1) were collected during oceanographic cruises of the ,oo

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273 bution of initial particles, prior to their possible aggregation in the water column or the sediment. The mineralogical composition of 136 samples was determined by X-ray diffractometry, using the oriented paste method (Philips scanning angle 2-32 ° 20, Cu-K~, monochromator [8]). In our data set, we have included results taken from previous studies in this area by Biscaye [2] but not those of Zimmerman [6]. Although both of these authors measured peak areas on X-ray diffractograms of only surface sediments, Zimmerman [6] analyzed the < 37/~m fraction, whereas Biscaye's and our data are based on the < 2/~m fraction. In this study, we measured X-ray peak heights and our values were obtained by averaging the data of samples from the entire Holocene period. The weighting factors and calculation scheme to obtain percentages is the same as those of Biscaye

[2]. For the same 136 samples, using the X-ray fluorescence technique (PW-1400 Philips spectrometer with Digital Mink II computer interfaced), we measured the content of the following elements in dried and ground powders: Na, Mg, A1, Si, P, S, CI, K, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, As, Br, Rb and Nb. By this method, we identified the main distributions of some recurrent components in the cores [13]. Based on this preliminary compositional study, we selected about 40 samples as the most representative of the total set. On those 40 samples, we used instrumental neutron activation analysis (INAA), for determinations of Sc, Sb, La, Ce, Nd, Sm, Eu, Tb, Lu, Hf, Th and Ta concentrations. Similarly, the surface water planktonic foraminifera samples were analyzed [10], to verify the blank levels of these elements in CaCO 3, thereby validating the calculation of CFB from analyses of bulk sediment samples. Analysis of the strontium isotope system (~7Sr/86Sr and 87Rb/86Sr) of the < 2 /~m size fraction were performed on ten decarbonated, surface sediment samples, by mass spectrometry (Cameca TSN-206 S) by N. Clauer (CNRS-Centre de S6dimentologie et G6ochimie de la Surface, Strasbourg, France). We used also the isotope data set from Dasch [14] although these data have only an indicative value because they were obtained on the bulk decarbonated samples, rather

than the < 2 /~m fraction as in our data set. Stratigraphic control of the Holocene boundaries in the sedimentary column was provided by a study of planktonic foraminifera [8,10]. In 20% of these planktonic foraminifera samples, the oxygene isotope stratigraphy was done by J.C. Duplessy (Centre des Faibles Radioactivit6s CNRS, Gif sur Yvette, France) [11]. Additional stratigraphic control was provided by an ashlayer, visually observed, which marks the horizon at 9800 yr B.P. [12]. These time datum levels were used to calculate accumulation rates. Study of living plankton samples was done by Pujol [10], in order to ascertain the present distribution of planktonic foraminifera. These were compared with the planktonic distributions observed in Holocene sediments to validate of the assumption that surface samples had been recovered. In this study, we only used the data for samples in which the comparison showed satisfactory resemblances (the coring technique may change the length of the Holocene sediments: we estimate this error smaller than 10%). Finally, in order to augment our data, we used various kinds of data from 44 samples discussed in the literature [15-20], for which locations are plotted in Fig. 1. 3. Results Data on lithology, trace and major elements and minerals are available in Grousset [8] and can be obtained from us: we summarize here these results as well as the Sr isotope ratios, granulometry and accumulation rates.

3.1. Mineralogy data Two minerals are particularly useful indicators of provenance of the MAR recent sediments, the clay minerals: smectite and chlorite. For these minerals, we also show the data from Biscaye [2] on our figures. The difference between the data sets is only that of measuring peak heights vs. areas, which accounts for the differences in absolute values. --Smectite (Fig. 2a): This mineral is the only one present on the Icelandic shelves and slopes where it comprises -- 100% of clays. Its concentration decreases southwards to 45°N, where it is

274 - - C h l o r i t e (Fig. 2b): The highest contents are observed in the northwest Atlantic Basin and in the Norwegian Basin, near the Scandinavia Shield. Between Iceland and the Azores chlorite contents increase progressively, but decrease from the northwest Atlantic abyssal plains ( - - 4 0 % ) to the

only --10%. A north-south gradient is clearly evident in the distribution pattern of this mineral. N o t e that, although the absolute concentrations differ because of differences in methodology, the north-south gradient in Biscaye's data [2] is approximatively the same as that in our data.

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275

northeast Spanish Abyssal Plain ( = 10-20%): this distribution pattern shows a west to east gradient, which is perpendicular to the gradient of smectite concentrations. Again, Biscaye's data [2], taken by themselves, show different absolute concentrations but same gradient. --Illite and quartz: These minerals patterns are approximately the same as that for chlorite (west to east gradient), but with a weaker gradient. - - S e c o n d a r y mineral: The concentrations of plagioclase feldspar, like smectites, also decrease in a north to south direction along the MAR.

(GFZ) and the Azores. Fig. 3b shows that the rubidium and chlorite contents are fairly well correlated (~ = 0.50). - - R E E elements: We analyzed lanthanides in particular in the " 4 5 0 N '' area ( 4 3 - 4 7 ° N , 2 3 30°W). The data are normalized to those of chondrites and shales (Fig. 4a, b). On this figure, we also show our results on Icelandic Tertiary basalts and tholeiitic submarine rocks dredged at 4 5 ° N in the Rift Valley. We observe a positive europium anomaly, similar to the Icelandic one. N o cerium anomaly is observed. - - S o m e trace elements (Ta, Sc, Co, Fe and Ti), exhibit a distribution of decreasing concentrations from north to south similar to that of smectite, but the data, particularly for Ta and Sc, are insufficient to accurately draw gradients for these elements. - - M n , Cu, Ni, Ba, Co and Zn: These six elements exhibit a slight increase of their concentrations, only in the Ridge area. The results are addressed in Grousset [8].

3.2. Elemental data From a large data set, entirely available in Grousset [8], only the most instructive will be presented here: - - R u b i d i u m pattern: In Fig. 3a, we note high Rb abundances close to the continents (above 90 ppm), decreasing to < 15 p p m around Iceland and the Faeroes. In the 4 0 - 5 0 ° N range, we observe an extension of high Rb contents from the West Basin, eastward to about 20 ° W. This west to east gradient, similar to that previously noted for chlorite, is also observed for Zr in a latitudinal band located between the Gibbs Fracture Zone

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276 C. Sample . C. chondriles

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plotted (Fig. 5a, white rectangles), in the context of the Dasch [14] values (black stars). Despite differences in grain size (Dasch: bulk sediment; this work: < 2 ffm), these two sets of data appear to be mutually consistent. We observe also a north-south gradient in the 87Sr/g6Sr ratios of our data. On the Icelandic slope, gVSr/86Sr = 0.70634

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278 TABLE l

TABLE 2

Mean grain size range (p-m) from Iceland to the Azores Core

Latitude

Depth (m)

Mean grain-size range (/~m)

KS7715 KS7706 KS7902 KS7921

63°N 62°N 55°N 45 ° N

1040 1420 2600 2075

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Non-carbonate Holocene accumulation rates (A.R., in gcm 2 kyr - 1) Core

H (cm)

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0.25 0.25 0.20 0.20 0.29

13.1 9.5 3.5 8.7 3.2

37 73 80 90 67 81 82 36 61 79 59 72

0.42 0.29 0.28 0.23 0.34 0.33 0.21 0.54 0.5l 0.24 0.27 0.33

1.7 1.3 1+3 1.1 2.3 2.8 2.7 1.7 0.9 4.0 3.6 0.8

69

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KS7714 KS7903 KS7904 KS7902 KS7901

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Chaucer.Faraday Ridge area 3.4. G r a n u l o m e t r i c d a t a

We observe a progressive decrease of the m e a n particle grain size from the Icelandic slopes to the Azores, similar to the gradients of smectite a n d the 87Sr/86Sr ratios (Table 1). There are no significant variations in grain size in the west-east direction. 3.5. A c c u m u l a t i o n rates

KS7914 KS7915 KS7916 KS7917 KS7918 KS7920 KS7921 KS7923 KS7924 KS7925 KS7926 KS7928

30 38 50 21 63 19 46 42 33 35 65 25

Noroit Seamount

Sediment a c c u m u l a t i o n rates are calculated here only for the carbonate-free fraction. A c c u m u l a t i o n rates are calculated as g cm 2 kyr-1 from the length of sediment between the 10,000 yr B.P. level (given by p l a n k t o n i c a n d tephrochronologic analyses a n d the surface, corrected for porosity a n d assuming a sediment grain density of 2.65 g cm -3. The a c c u m u l a t i o n rates decrease linearly from the n o r t h (13.1 g c m - 2 k y r - i in the vicinity of Iceland), to the south (1 g cm 2 kyr 1 at the Azores, near 4 3 ° N ) (Table 2 a n d Fig. 6a, b).

KS7929

65

H represents the thickness of the Holocene deposit and W is equal to the ratio: (B - D ) / D where B = bulk sediment weight and D = dry sediment weight.

The second group consists of those tracers characterized by a west to east gradient, i.e., chlorite and rubidium. We will discuss the m e a n i n g of these gradients successively in terms of origins, transport mechanisms and a c c u m u l a t i o n rates.

4. Discussion 4.1. O r i g i n s

F r o m results presented above, it is possible to group the various tracers and other data into two m a i n patterns. The first group consists of those tracers exhibiting a north to south gradient primarily along the M A R . This is the case for smectite, for STSr/S6Sr a n d for grain size. It is also the case for mass a c c u m u l a t i o n rates: though the n o r t h to south direction is the p r i m a r y orientation of the stations (i.e., it is difficult to see if there are east-west gradients for example), we assume this, because the mass a c c u m u l a t i o n rates are well correlated with smectite contents ( ? = 0 . 9 1 ) and ~7Sr/86Sr ratios ( ? = 0 . 8 4 ) , both parameters for which we have a large data set.

W e s t - e a s t gradients. The latitudinal b a n d in which this gradient is most obvious is located between --- 4 0 ° N a n d --- 6 0 ° N . Because the highest values are in the western part of this area, we regard the n o r t h e r n U n i t e d States a n d the C a n a d i a n Shield as a source. Chlorite, illite a n d r u b i d i u m are a b u n d a n t on the rocks a n d soils of the C a n a d i a n Shield a n d in the s u r r o u n d i n g sediments [7,21]. These tracers are also relatively a b u n d a n t on the E u r o p e a n c o n t i n e n t , but, aside from the high chlorite in the N o r w e g i a n Sea they do not contribute to the west-east gradient in the 4 0 - 6 0 ° N . latitudinal b a n d . Finally, these tracers are absent

279 GREENLAND

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MAR, and the volcanic islands (Iceland and Faeroes), southward along the MAR? The possibilities would appear to be gravity currents (and other down-slopes processes), surface, intermediate and bottom currents and eolian transport.

Transport mechanisms for the North American material. In Figs. 2b and 3a, we observe that these

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or very poor, on the Azores and Iceland rocks and soils.

North-south gradients. These gradients are observed primarily along the MAR. Unfortunately, we have no data from the Irminger Basin (between the M A R and Greenland), so we will only discuss the east side of the M A R from Iceland to the GFZ. Between the G F Z and the Azores, we have data on the both sides of the Ridge. The highest values of the tracers exhibiting north to south gradients, e.g. smectite and trace elements such as Ta or Sc, are located around Iceland and the Faeroe Islands [8]. These samples exhibiting the highest smectite, Ta and Sc contents also have the lowest 87Sr/86Sr ratios. The northsouth orientation of these gradients suggest Iceland and the Faeroe islands as the provenance of sediments bearing these signatures. In this region, these islands are the only potential sources to have such high concentrations of smectite [8,22,23], and, because they consist of entirely basaltic rocks [24], they have low Sr isotope ratios. The volcanic Azores, also a potential source of smectite and low 87Sr/868r, appear not to contribute significantly to this trend. The North American and European continents are not characterized by sources of low 87Sr/86Sr and high smectite, Sc and Ta.

gradients traverse the ridge axis, i.e., material of Canadian origin is found east of the MAR. Therefore, we can eliminate from consideration processes operating below a water depth of 2000 m, because the Ridge is sufficiently continuous to act as a true physical barrier below that depth. As far as surface currents are concerned, the major influences are the Gulf Stream and the North Atlantic Drift (Fig. 7). There are, however, two problems with attributing the transport of North American material to these surface currents. First, the material from the North American continent would be transported following along a south-west to north-east axis [22] and this is not similar to the major west-east overflow axis we observed in Figs. 2b and 3a. Second, the problem of invoking surface currents is primarily one of particle settling rates, relative to eastward surface current transport rates. The existence of fast settling mechanisms precludes a long-range eastward transport of suspended particles by surface water movements. From the studies of Deuser [25] and of Bruland and Coale [26], we may assume that there is a good inverse correlation between primary productivity and clay particle residence time. According to Raymont [27], an important primary

4.2. Transport mechanisms What are the transport mechanisms for the particles derived from these two major sources: the North America westward to and beyond the

Fig. 8. Schematic of deep hydrological circulations in the North Atlantic (dark line: after Worthington [36]; Arrows 1 and 2 are unpublished data from Gould). The grey area is the Ridge region between 43 and 47°N.

280

mented [31,32 and references there in]. In 1970, Folger [33] demonstrated long-distance wind transport. He collected aerosols between New Foundland and the British isles: these materials consisted of clay aggregates, quartz, lacustrine diatoms, fungi and phytoliths, which are obviously continental-derived. Thus, the wind could be responsible for the distribution of the east-west gradient tracers chlorite and rubidium (Figs. 2b, 3a; section 3.1). This situation is in good agreement with the present atmospheric patterns in the region between 40 and 60°N [34]. This is in agreement with Windom et al.'s [7] interpretation, based primarily on Biscaye's illite data [2] in the North Atlantic. This, however, is at variance with Biscaye's own interpretation who concluded that the MAR acted as a barrier to the distribution of North American material. This was based on the distribution of several minerals, but especially that of mixed layer clay minerals [2, Fig. 9]. We offer the following rationale for these observation based on personal communication with Biscaye. The distribution of mixed layer minerals [2], indicates a northwesterly source in that they are sharply restricted to the west of the MAR and increase in concentration to the north. If their origin were in the soils of the extreme north (Greenland, Baffin Islands, etc.), that is, north of the latitude of the westerlies, their south-

productivity has been observed along the shelves of the North American continent ( = 250 mg C m-= day-]). A comparison of the Bruland and Coale [26] residence time range versus productivity obtained in the Pacific Ocean (2.5-60 days) with Raymont's data, suggests particle residence times in the northwest Atlantic of the order of a week. Given the average speed of the surface currents in this area ( < 1 m s - l ) , a transport of sediment particles from North American continent to the MAR (--2000 km) would require at least one month, much more than the probable particle residence time in the surface water column. According to the discussion by McCave [28], we may neglect the contribution of small particles advected in the intermediate water masses [29,30], because their scavenging by large settling aggregates would be less than 10% of the total small particles. The influence of bottom currents, related to the Norwegian Overflow, can also be dismissed because of their north to south dispersion axis, which is perpendicular to the observed west to east gradients. The most likely candidate, then, as a mechanism for the transport of material having a North American origin, is eolian transport. The transport capacity by wind over long distances is well docu87Sr 86 Sr

0.730 j

07Z5.

0720 -

Province ~

~

071507920

7901 7~02 ~ ~7904

o7712

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' 75

Fig. 9. Relationship between SVSr/86Sr and 1/Sr × 10 3. A mixing line joins the Iceland and the North American province.

281 TABLE3 Estimation ofthe REE mixingratios(REE, inppm) inthecore KS-7920(45°N) Iceland (our data) Shales (A) [47] Mixtures (M) Sample a (S)

La 28.2 41 35.6 44.4

Ce 57.2 83 72.1 59.4

Sm 7.4 7.5 7.5 7.0

Eu 3.5 1.61 2.4 2.6

Tb 1.3 1.23 1.25 1.32

Yb 3.1 3.53 3.3 3.9

Lu 0.40 0.61 0.52 0.45

S/M ratio Divergence

1.25 25%

0.82 18%

0.94 6%

1.1 10%

1.05 5%

1.18 18%

0.84 16%

a KS7920.

ward transport could be by the south-flowing bottom currents (Denmark Straits Overflow Water, Western Boundary Under Current, Labrador Current, etc.), all restricted to the west of the MAR. At the same time non-mixed-layer-bearing NorthAmerican material (chlorite, Rb-bearing illite, etc.) could be transported eastward across the North American Basin and the M A R by the westerlies.

Transport mechanisms for the Icelandic-Faeroe materials. The north to south gradients of smectite, 87Sr/86Sr, Ta and Sc [13] imply transport southward from Iceland-Faeroes. What transport mechanisms could then be responsible for this southward transport, which is effective to about 45°N, i.e. 2000 km from the source? As discussed above, atmospheric circulation could not possibly be responsible for this transport, because of the prevailing west to east mean direction [34]. Also surface currents transport cannot be called upon as transport mechanism because the North Atlantic (Gulf Stream) gyre is from southwest to northeast in this part of the North Atlantic [35] (Fig. 7). The only transport mechanism flowing from north to south in the study area is transport by deep bottom currents. This southward circulation is well recognized between the Iceland-Faeroe sill and the latitude of the GFZ. Worthington [36] described the north to south movement of the b o t t o m water (Norwegian Overflow) in the Icelandic Basin (Fig. 8). Elsewhere in this basin, bottom-current measurements 1 m above the bottom (albeit of short duration) show the same north to south direction [37,38]. However, in the Ridge area, between the G F Z and the Azores, the deep circulation is not very well understood. In their

hydrological study of the North Atlantic, Wunsch and Grant [39], observed a north to south movement of the deep currents in the water column, in the Ridge region both east and west of the MAR. Finally, on the east side of the Ridge, between the G F Z and the Azores, two 9-months-duration bottom-current measurement 1 m above the bottom also show a southerly current direction (Gould, personal communication). At these sites, near the locations of cores KS7917 and KS7926 (Fig. 1), the bottom water flows southwards at 1 cm s -1 (net mean velocity). The activity of bottom currents induces resuspension of sedimentary particles, which form a nepheloid layer above the bottom. The phenomenon has been observed in association with m a n y boundary currents in the abyssal Atlantic Ocean and has been proposed to explain the horizontal displacement of sediment [40-44]. Such nepheloid layers have also been identified by Jones et al. [43] in the Icelandic Basin. We propose this mechanism to explain the distribution of the particles as identified in this study, in the ridge region. This southward transport of Icelandic-Faeroe material would explain the progressive decrease of smectite content and the progressive increase of 87Sr/86Sr, by a continuous mixing of these sediments derived from basic rocks with those with a provenance of acidic continental shields.

4. 3. Accumulation rates Accumulation rates (A.R.) of terrigenous sediments. The discussion of accumulation rates deals only with the non-carbonate components of the sediments. A.R. decrease progressively from the North Icelandic Basin (A.R. = 13.1 g cm -2 k y r - l ) , to

282 the north of Azores, where they are only about 1.3 g cm 2 kyr-1 (Fig. 6a). There is an order of magnitude decrease between these two areas and this decrease is linear: 0.5 g c m - 2 kyr -1 every 100 km (Fig. 6b). This linear decrease indicates a conic type of dilution of the sediment load. In the North Azores area, the A.R. are in a range between 0.9 and 1.3 g cm -2 kyr -1. These values are very similar to the mean value extrapolated by Turekian [1] in this area.

Partitioning between sources. As demonstrated previously, both North American and Icelandic materials reach the ridge region in the 4 0 - 6 0 ° N latitude band, where they are mixed. In order to evaluate their partitioning, we propose three independent approaches, using data at 4 5 ° N as an example. The first approach is a calculation based on isotopic ratios of Sr with a control by the REE. The second is based on multivariate statistics, and the third by comparing the mean Ridge A.R. with that on an abyssal seamount. (1) Calculation of the mixing ratio based on Sr isotope ratios. As is observed in Figs. 5a-9, sediments represent a mixing of Icelandic material with SVSr/S6Sr = 0.7033 (with Sr = 87/xg g-1 and Rb = 11 ~tg g-~ in the soils [8]) and of American continental materials of STSr/S6Sr=0.722 [45] (with Sr = 283 /~g g-~ [46] and Rb = 110 /tg g-~ [46]). Evaluation of the mixing for a given sediment sample is given by the linear equation: (SVSr/S6Sr)~- C~ = X~," (SVSr/86 St)a-[Sr]a + (1 - X)i" (SVSr/S6 Sr)i' [Sr]i where C = seawater influence correction, X = contribution, and subscripts s = sediment sample, a = American material, i = Icelandic material. Use of this equation for the core KS7920 (at about 45°N), where S V S r / 8 6 S r = 0.71304, yields the conclusion that the sediment is comprised of 42% of material of Icelandic origin and about 58% of material of North American derivation. This evaluation is confirmed by t h e 878r/86Sr versus L a / Y b relationship (Grousset, in preparation). These proportionality constants may be applied to the REE concentrations of the same sample, in order to test their validity. We may calculate the theoretical contents of a mixing of Icelandic and North American materials as follows, after a

method developed by Bonnot-Courtois [47]: M = 42% REE (Iceland) + 58% REE (North America) If S represents the REE (CFB) of the KS7920 sample, the S / M ratio must be equal to 1 for each REE if the 42 : 58 partitioning is correct (Table 3). Since the S / M ratio does not depart from 1 by more than 25%, we feel this tends to confirm the -- 40:60% partitioning given by the strontium isotope data at 45°N. (2) Approach based on multivariate statistics. Multivariate statistics have been applied with success to East Pacific Recent sediments by Eeinen and Pisias [48]. These authors used this approach to obtain accurate end-member evaluations. One of us (F.G.), used a similar approach (factor analysis in R and Q modes) in the Icelandic and the Norwegian Basins, in which we quantified the partitioning between a biogenic factor, a "basic" factor (Iceland-derived) and an "acidic" factor (continent-derived materials) [49]. One of us [8], applied this statistical approach to the data of the present study, on the set of data obtained by neutron activation (see section 2). It appears that samples of the 45 ° N area are characterized by an approximate equality in weight of the "acidic" factor (here, the North American products) and of the "basic" factor (Icelandic products). By this approach it appears that each factor represents 50% of the carbonate-free sediment in this site. This approach, statistical factors and graphic projections are extensively described in Grousset [8]. (3) Ridge and seamount A.R. comparison. A third means of evaluation is possible by comparing A.R. in two areas: (a) the Noroit Seamount (Biscaye Abyssal Plain, 47.18°N, 15.04°W), and (b) the M A R (between 4 3 - 4 7 ° N and 23 29.30°W). Arguments favoring the selection of the Noroit Seamount (site A) are: - - T h e long distance existing between this seamount and the continental sources, both east and west, as well as the distance from the Ridge axis (500 km), - - T h e westerly winds and surface current directions imply that the seamount should receive a minimum of material from Europe and African sources, - - T h e top of the seamount lies 700 m above

283

the ocean floor and consequently must not have been much affected by sporadic continental turbidites. We further assume that, contrary to the ridge situation, the Noroit Seamount is almost unaffected by the advected flux from Iceland. This is confirmed by the multivariate statistical approach which shows clearly a primarily acidic continental imprint in the seamount deposits [8]. The samples from this site plot within the acidic factor, along with the continental shale samples and with the acidic tracers: Rb, Si, etc. This rationale could be applied to Europe too, but we have no means of distinction between European and North American contributions. So, if the seamount receives an European input, our evaluation of the North American contribution will be a maximum value. - - A c c o r d i n g to the observations of Raymont [27], primary productivities are more or less the same over both regions of interest (range: 150-250 mg C cm -2 day-a). So, referring to Deuser et al.'s [25] observations, we think that the alumino-silicate accumulation rates (derived from the surface) are quite the same in both regions. From these arguments, it may be assumed that the sampling location at the top of the seamount is essentially subject to the settling phase of the sediment falling from the overlying system (eolian a n d / o r surface watermass transports). The region of the M A R between 4 3 - 4 7 ° N and 23°-29.30°W (site B; Fig. 1) was selected because A is located in the same latitudinal range and both sites are subject to the same westerly wind regime. Thus, except for the southward advective flux by bottom currents from Iceland, this region is subject to about the same vertical flux as the Noroit Seamount. The A.R. calculated at the two sites showed the following values (Fig. 6a):

(A) is 0.7 g cm -2 kyr -1. Given our assumptions, then this difference represents the flux advected from the Icelandic system, while the Noroit Seamount A.R., 1.6 g cm -2 kyr -1, represents, as discussed above, the maximum value of the vertical flux from North America. This evaluation gives a partitioning between the Icelandic and the North American contributions of = 30% to -- 70%. We feel that the data obtained by this comparison of the two sites are in reasonable agreement with the two previous evaluations (1) and (2) obtained by independent approaches. The results of the three approaches are summarized in Table 4. These values concern only the 4 5 ° N area. Now, we can summarize the change in the relative contributions of the different sediment sources along the Ridge as follows: on the Icelandic continental shelf and upper slope, sediments are entirely derived from Iceland (Icelandic contribution: ---100%, North American contribution: = 0%). The Icelandic contribution decreases progressively southward along the Ridge. In the Azores region, it is estimated to be 30-55%, while the North American contribution estimates range from 70 to 45 %. The more precise evaluation of the partitioning given by multivariate statistical analysis of the larger data set (2) yields the lowest estimate for the North American contribution. If we take as an example the 45°N area, the partitioning is found to be ---50/50%. The lowest calculated bulk accumulation rates for this area are close to 1 g c m -2 kyr 1, the value found by Turekian [1] in this area. In terms of this accumulation rate, the North American eolian contribution would be 0.5 g cm -2 kyr i (50%) and the Icelandic deep water advected contribution 0.5 g cm 2 k y r - t (50%). These estimates should be taken as minimum

- - Site A (Noroit Seamount): A,R. =1.6 g c m -2 kyr -1 - - Site B (Ridge): the mean A.R. of the 8 cores located in this area (KS7915 =1.3, KS7916 =1.2, KS7918: 2.3, KS7920: 2.8 KS7921: 2.2, KS7924: 0.9, KS7925: 4.0, KS7926: 3 . 6 ) = 2 . 3 g c m 2 kyr 1

TABLE 4

We observe a slight scattering in this data set that we cannot explain: it may be due to the coring technique, which can sometimes blow the first centimeters of the sediment. The difference between the Ridge A.R. (B) and that at the seamount

Contribution of the main sources. This table presents the various percentages of the contributions of the two main sources to the 45°N area, obtained by the three independent approaches used in this study Method

North American contribution

Icelandic contribution

(1) Sr isotope (2) Multivariate statistics (3) Two-sites comparison

60% 45-55% 70%

40% 55-45% 30%

284 Eolian Transport

values as they are based upon the lowest calculated bulk accumulation rate for this region.

4.4. A diagnostic' model for accumulation rates in the North Atlantic' Ridge sedimentary regime In this study, we have treated various components of the sediments as tracers. A first-order relationship between the source regions and the sediment assemblages was proposed. In Fig. 10, we introduce the three source regions (S 1, S> $3), together with a spectrum of fluxes from various transport mechanisms (~5). We will discuss below, in sequence, each parameter introduced in the model, in order to quantify each of them. Fluxes will be differentiated on the basis of various processes which were partially described by Chester and Aston [15] at the scale of the global ocean. We have: (1) The vertical flux ( ~ , ) . This first type of flux includes all detrital particles, transported by rivers, by wind (~e), by surface water masses ( ~ ) , which have ultimately settled to the bottom. Such material encompasses particles floating in the sea surface water and also particles found in the deeper water column. (2) The b o t t o m advective flux (qSa). This second type of flux also comprises material ultimately derived from emergent areas. However, this material settles first on the continental shelves and slopes, then feeds the deeper part of the water column by down slope processes [2]. It is essentially dispersed in the deep ocean by b o t t o m currents. As this mode of transport is quasi-horizontal, we incorporate materials derived by such processes in the advective flux term. The ridge flux (qSr) represents the sum of q~a and eOv. The hydrothermal flux ( ~ h ) is negligible. We summarize below, the numerical values (in g cm : kyr-~) which were determined in this study for the various fluxes at the 45°N area (Fig. 10): q~r = 2.3 d0,.= 1.6 q?a = 0.7 q)~ + doe = 1.6 • ~ = 0.5 0/,t, ~ and ~h are non determined in this study.

Sourc

Source ] ( r i f t activity )

Fig. 10. Diagnostic model for the studied area. Three major sources are S l, S: and S3. They supply specific flux materials: • ~ and ~t = down slope process fluxes; ~ = surface current flux; ~,~ = advective bottom flux; ~e = eolian flux: 113 r ~ rift flux. They are the main components of ~v (vertical flux) and of ~ (advective flux).

To resolve the undetermined fluxes shown above, an integrated approach should be made with an appropriate grid sampling of tracer distributions.

4.5. Departures from the Holocene uniformity Are the present-day hydrological and atmospherical circulations representative of conditions which have prevailed during the entire Holocene period? The question which can be raised for our data, is that the values displayed are averaged values. By averaging, we may have smoothed any signal of inner variability in the sedimentary regime. However, we argue as follows for consideration of the overall sampling as representative of the entire Holocene regime. As we stated before, two main environmental conditions control the sediment accumulation rate: the deep current regime and the prevailing wind pattern. - - D u p l e s s y et al. [50], using 8~3C measurements concluded that during the Holocene, the mean field of deep currents in the N o r t h Atlantic has been almost constant. - - A c c o r d i n g to L a m b [34] and L a m b and W o o d r o f f e [51], since 8500 yr B.P. the global

285 atmospheric circulation has not changed drastically in the northern hemisphere between 40 ° and 60°N. The different mean wind regimes reconstituted by Lamb [34] at four different periods (8500, 6000, 4000 and 2500 yr B.P.), clearly show that the westerlies were constant over the North Atlantic Ridge region. But, we have to consider here an important drawback in our diagnostic model. If we compare the Holocene eolian contribution evaluated above, (0.5 g cm -2 kyr 1) with the present time measured delivery, which is about 0.1 g cm 2 kyr 1, estimated by Goldberg [3] and measured by Chester and Aston [15] and BuatMenard and Chesselet [31], it appears that the eolian flux has been more intense during some parts of the Holocene than at present. The observed present time eolian delivery appears to be a factor of 5 lower than the integrated Holocene aeolian delivery. It is worth noting here that, in the North Central Pacific, for the same latitude, in the same westerly regime, Rea et al. [52] have observed sporadic, strong increases of eolian input (factor of 7) in the early part of the Holocene (6000 yr B.P.). The first part of the Holocene corresponds to the optimum climatic (the more arid period) and it is possible to assume that, in our area, the eolian flux reached sporadically 1 g cm 2 kyr-1, without pattern changes. This would explain the high value obtained for the entire Holocene. Looking again at the Lamb's wind pattern analyses for 8500 yr B.P., it may be argued that the high delivery at that time could be related to a high input of material derived from the desertic part of Africa, which was controlled by the longitudinal displacement of the Azores highpressure zone. Our 87Sr/86Sr ratios, however, counter these arguments. Indeed, these ratios vary greatly between the Canadian source (0.722 [45]) and the African source (0.715-0.721), with a narrower range in aerosols above Africa and Barbados (0.715-0.717 [53]). Such a large difference between 87Sr/86Sr ratios in eolian material derived from the Canadian and African provinces are not seen in our data (Fig. 5a). Meanwhile, because the stratigraphic, latitudinal and eolian similarities between our situation, and that of Rea's data in the Pacific, we claim that the same interpretation can be applied to explain changes in the Holocene eolian delivery, making a strong case for a variable climatic forcing, for

which the consequences are recorded in the MAR Holocene sediments.

5. Conclusions Non-carbonate particles, settling on the MidAtlantic Ridge from the Azores to Iceland during the Holocene, have been studied using the sampling grid shown in Fig. 1, using mineralogical and geochemical tracer distributions. Accumulation rates decrease linearly from the Iceland shelf and slope (13 g cm -z kyr-1), to the Azores area (1 g c m -2 kyr 1). North-south gradients of 87Sr/86Sr ratios and smectite contents, and west-east gradients of rubidium and chlorite contents allow one to identify the two main sources of particles: the Icelandic system and the North American continent. On the Icelandic slopes, the Icelandic contribution represents the entire input. This contribution decreases progressively toward the Azores region, where it accounts for 30-50% of the carbonate-free Holocene deposits. This material is transported from the Icelandic area by deep and bottom currents. At the same time, the proportion of North American contribution increases continuously and, at 45°N, it represents 50-70% of the detrital input. This is 5 times the present-day aeolian flux. Climatic change must be the parameter explaining the decrease in aeolian sediment delivery.

Acknowledgements This paper consists largely of work from the doctoral (Doctorat d'Etat) dissertation of F.G. at the University of Bordeaux I, France. We are indebted to Pierre E. Biscaye, for his contributions to this paper from criticisms of the thesis and participation in the thesis committee, to many discussions of the data and concepts, to critical reviews of several drafts of this paper. We are also indebted to the anonymous referee and to J.F. Minster for their suggestions to improve the manuscript. We thank C. Latouche, for allowing us to carry out X-ray diffractometry and fluorescence analyses at the I.G.B.A. laboratory and for his criticisms and encouragements. We are very grateful to C.E. Lambert and A. Isley for their reviews of early draft of the manuscript. We thank P. Buat-Menard, J.C. Brun-Cottan, C.H. Lang-

286 m u i r , C. P u j o l a n d M, S a r n t h e i n for t h e i r s u g g e s t i o n s a n d c o m m e n t s . W e t h a n k N . C l a u e r a n d J.C, D u p l e s s y for t h e i r c o o p e r a t i o n in t h e m a s s s p e c -

I4

t r o m e t r y a n d M. Treuil a n d J,L. J o r o n f o r t h e i r h e l p , in t h e n e u t r o n a c t i v a t i o n m e a s u r e m e n t s , int e r p r e t a t i o n s of the d a t a a n d their discussions. The SUE"

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18

P r o g r a m : " G e o l o g i e et G e o p h y s i q u e d e s O c e a n s " .

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