Temporal and spatial variability of sedimentation rates

Sedimentation rates in all cores are ba ed on analogue ages of oxygen i otope events and available AMS age , a pre ented in Table 4.1. During the last 30,000 years, ¹⁴C ages do not correspond to true (calendar) ages, which is demonstrated by discrepancies between radiocar-bon ages and age obtained by other methods including annual chronologies, ba ed on tree-rings (e.g. Stuiver et al. 1986, Becker & Kromer 1993) or varved sediments (e.g. Lotter et al.

1992), and U-Th ages (Bard et al. 1990). Mostly, ¹⁴C age are systematically younger, with a maximum difference of 3,500 year prior to 13,200 ¹⁴C yrs. B.P. (Bard et al. 1990). Changes in the production of ¹⁴C in the atmo phere eau ed by variations of the geomagnetic field strength appear to be the dominant controlling factor to account for these differences (Stuiver et al. 1991, Mazaud et al. 1992), be ides changes in the C0₂ exchange between the ocean and the atmo phere (Sarnthein et al. 1994). The relation hip between ¹⁴C age and calendar years i tele copic and partly non-linear due to the presence of major "¹⁴C plateaus"; therefore sedimen-tation rate were calculated in the domain of calendar year only. ¹⁴C ages were converted into calendar years according to Stuiver & Braziunas ( 1993), following the tree-ring chronology of Stuiver et al. (1991) and Becker & Kromer ( 1993) for the Holocene and the Uffh datings of Bard et al. ( 1990, 1993) for Termination I and oxygen isotope stage 2.

Sedimentation rates are con istently high (3-30 cmlky) in the tudied cores close to the ridge axis, compared to a regional average of 1.6 crnlky (G.A. Auffret, written comm. 1995).

This may be due to local ediment ponding and may al o include the effects of enhanced biogenic productivity and/or preservation and of higher detrital input derived locally from the ridge axis. Among the four cores studied, lowest rates occur in core Ac.91 KS04, which is ituated in a mall ba in ju t outside the MAR rift valley. Higher rate occur inside the rift valley in cores KFI3 and KF09, more elevated rate in the latter core are con i tent with the regional pattern of higher rates in the outhern part of the study area (B.Dennielou, pers. comm.). Very high edimentation rate in core KF 16, which are an order of magnitude higher than in the other core , are related to the particular core location in the deepe t part of a basin in the central area of the 38°05'N fracture zone. This basin appears to act as a ediment trap, implying ignificant lateral ediment upply at thi core site, e.g. including a fine-grained layer with abundant

·erpentine, which is inferred to be of allochthonou origin (cf. Chapter 9).

Sedimentation rates are higher during i otope stage 2 and the early Termination I than in the Holocene. Thi a umption may be valid even though some lo of surface sediment i typical of box core and has certainly occurred in cores KF 13 and Ac.91 KS04. However, it is rather improbable that a ub tantial Holocene sediment section ha been entirely lost during the coring proces in core Ac.91 KS04, since the oxidized urface layer i well preserved. For calculation of accumulation rate near the surface of core KF 13, con. tant sedimentation rates of 6.6 crnlky were a umed for the late Holocene in core KF13. This would imply a loss of 27 cm at the core urface, which is till reasonable for box cores.

25

Ac.91 KS04 (38°05.45N 30°3 5.84W I 2 183 m water depth)

Event Depth (cm)

1.1

⁴

2.2 (LGM) 82

3.1

¹⁷⁵

3.31

263

3.33

278

4.2 334

5.1

377

Age (k_r cal.) 9.8

18.3 29.5 50.2 55.5 65.2 79.25

Sed. rate (cm/~)

9.2 8.3

4.3 (3.8*) 2.8

5.9 (5.2*) 3.0 (2.4*)

*

excluding volcanic ash layers

GEOFAR KF13 (37°34.698N 31 °50.53W I 2690 m water depth)

Event foraminiferal Depth '"C-Age Age Sed.rate

species (cm) (ky) (ky cal.) (cm/ky)

AMS-Age G.ruber 22 6.99±80 7.43

AM S-Age G.ruber 30 8.21±80 8.64 6.6±1.0

1.1

+

AMS-Age G.ruber 40 9.01±90 9.62 10.2±3.4

Younger Dry as 60 10.4 12.4 7.7±1.1

AMS-Age G.bulloides 73 12.23±130 14.2 7.2±0.5

AMS-Age G.bulloides 100 13.03±140 15.0 33.8±8.6

AMS-Age G.bulloides 110 13.52±160 16.75 5.7±0.9

AMS-Age G.bulloides 130 14.16±160 17.66 31.3±10.5

2.2 (LGM) 150 14.8 18.3 31.3±6.3

AMS-Age G.bulloides 170 16.16±210 19.66 14.7±2.7

AMS-Age G.bulloides 200 18.69±290 22.19 11.9±2.9

AMS-Age G.bulloides 220 20.20±340 23.7 13.2±3.9

AM S-Age G.bulloides 240 21.73±430 25.3 13.1±4.4

3.1

320? 26.00 29.5 19.0±1.7

3.3

510 50.2 9.2

4.22 560 60.4 4.9

GEOFAR KF16 (37 °59.94N 3l

⁰

07.70W I 3050

m

water depth)

Event Depth (cm)

1.1 200

YD

²⁵⁰

End Term. la 420

2.2

⁴⁹⁰

3.1 >800

Age (ky cal.) 9.8

12.4 17.1 18.3 29.5

Sed.rate (cm/ky) 20.4 (18. 7 *)

19.2 36.2 58.3

>28.6

*excluding allochtho-nous serpentine layer

GEOFAR KF09 (37°06.680N 32°17.208W I 2655 m water depth)

Event foraminiferal Depth , .. C-Age Age Sed.rate species (cm) (ky) (ky cal.) (cm/ky)

1.1 80 9.0 9.8 8.9

YD

^{130 10.3} 12.4 19.2

End Term. la 200 13.6 17.1 14.9

2.2

230 14.8 18.3 25.0

AM S-Age G.bulloides 320 17.91±260 21.41 28.9±2.2

AMS-Ag_e G.bulloides 380 20.95±380 24.45 19.7±3.4

AMS-Age G.bulloides 420 22.15±430 25.65 33.3±13.4

3.1 >470 26.00 29.5 >13.0

Table 4.1: Summary of age data for cores GEOFAR KF13, Ac.91 KS04, GEOFAR KF16 and GEOFAR KF09. Age control based on analogue ages of oxygen isotope events, and AMS ¹⁴C age in core KF13 and KF09. Surface sedimentation rates in cores Ac.91KS04 and KF13 not indicated, a ome surface sediment was certainly lost during the coring proce s.

The detailed AMS age profile in core KF13 allows to deduce short-term variability of sedimentation rates during Termination I. Between 13.0 and 13.5 ¹⁴C kyrs. B. P., the capacity of uch investigations is limited by potential artifacts of the conver ion from ¹⁴C year. into calendar year , because the non-linear relation hip between the two time cale is particularly pronounced in thi time interval, and the extent of ¹⁴C plateaus i yet unknown. A pul e of very high edimentation rates occurs immediately ub equent to the Last Glacial Maximum. AMS ages of 13,520 and 14,160 ¹⁴C yrs. B.P. indicate that this interval is coeval to Heinrich layer 1, which was AMS-dated at 13,490-14,590 ¹⁴C yrs. B.P. at DSDP Site 609 (Bond et al. 1992). No ice-rafted detritu wa ob erved in the coarse fraction, which i not

27

surprising because this core is located slightly south of the main IRD belt from 40-55°N in the Atlantic Ocean (Ruddiman 1977, Grou set et al. 1993). Based on geochemical data it is uggested that this interval was characterized by enhanced continentally-derived detrital input and possibly by higher productivity (cf. Chapter 8). The rapid drop in sedimentation rates above ( 100-110 cm) may be at least partially an artifact of the conversion from ¹⁴C- into calendar years. Therefore, the ,amplitude" of a second sedimentation pul e which apparently occurred between 12.2 and 13.0 ¹⁴C yrs. B.P. cannot be ascertained. For Isotope Stage 2, sedimentation rates are remarkably constant in core KF13, aside from higher rates in the early part of this stage; on the other hand, the AMS age data indicate highly variable rates in core KF09. Prior to stage 2, sedimentation rates drop in cores Ac.91 KS04 and KF 13 and rise slightly during tage 4 in the first core.

4.4 Conclusion

Generally, the nature of the isotope records indicates that continuous accumulation of pelagic sediments was perturbed by major turbidites and/or hiatu es. Thus the core selected for geochemical analysis should provide a reliable record of sedimentary fluxes during the last 30,000 to 80,000 years.

Im Dokument THOMAS RICHTER SEDIMENTARY FLUXES AT THE MID-ATLANTIC RIDGE (Seite 32-36)