PALEOCEANOGRAPHY, VOL. 13, NO. 4, PAGES , AUGUST 1998

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1 PALEOCEANOGRAPHIC CURRENTS PALEOCEANOGRAPHY, VOL. 13, NO. 4, PAGES , AUGUST 1998 What level of resolution is attainable Results of a spectrophotometer study in a deep-sea core? Mark R. Chapman and Nicholas J. Shackleton Godwin Institute for Quaternary Research, Department of Earth Sciences, University of Cambridge, Cambridge England, United Kingdom Abstract. A Minolta CM-2022 spectrophotometer has been used to characterize downcore fluctuations in sediment lightness and color in core NEAP15K, a 7-m core collected from the northeast Atlantic Ocean. High-resolution data series, measured using a 4-mm-diameter measurement spot and a 1-cm sampling interval, were generated along two independentracks down the core to investigate the statistical significance of fluctuations across the 2-20-cm-depth range. This small-scale variability is characterized by abrupt changes in the lightness and color of the sedimenthat are several orders of magnitude greater than the instrumental precision. Our results establish that significant information is preserved at the 1-cm scale in spite of bioturbation effects. These findings demonstrate that high-resolution studies using conventional paleoclimatic proxies have the potential to recover meaningful century-scale climate records in regions of the ocean where sedimentation rates exceed 10 cm per thousand years. The coherency of these downcore records also implies that the spectrophotometer is a powerful instrument for establishing precise centimeter-scale stratigraphicorrelations between cores. 1. Introduction Color is a fundamental sediment property. In sediment cores from the North Atlantic region the alternating light/dark pattern of carbonate-rich and clay-rich sediments evident within late Pliocene and Quaternary marine sequences provides a valuable, if somewhat imprecise, paleoclimatic proxy. High-resolution grayscale reflectance records have been used to study high-frequency climatic variability over the last glacial-interglacial climatic cycle [Bond et al., 1992; Cortijo et al., 1995]. During the last glacial period (oxygen isotope stages 2-4) the normal biogenic sedimentation pattern was interrupted by brief intervals of enhanced ice-rafted detritus deposition (the so-called "Heinrich events"). These paleoenvironmental changes were characterized by a switch from light carbonate-rich sediments to much darker sediments during ice-rafting episodes. The direct comparison of grayscale reflectance and fluctuations in the abundance of sinistrally coiled Neogloboquadrina pachyderrna (a polar planktonic foraminifer) made by Bond et al. [1992] demonstrated that sediment color changes can yield highresolution paleoclimatic information similar to that documented by the study of Greenland ice cores [Dansgaard et al., 1993; Taylor et al., 1993]. The goal of this study is to determine the smallest depth scale at which coherent variability exists in a deep-sea core and hence more accurately evaluate the maximum temporal resolution attainable within the sedimentary record. For this work we use a spectrophotometer that quantifies sediment lightness (approximately equivalent to grayscale lightness) and color variability (chromaticity) obtained from direct Copyright 1998 by the American Geophysical Union. Paper number 98PA /98/98PA analysis of sediment material. Our strategy is to determine down to what scale any observed fluctuations in sediment lightness and color are coherent across a 15-cm-wide section of marine core. 2. Materials and Methods Core NEAP15K was collected from the southern Gardar Drift (54øN, 28øW; 2848 m water depth) in a position situated well above the carbonate compensation depth [McCave, 1994]. Immediately after collection, material from the 15-cm square Kasten core was subsampled into trays (dimensions: 33 by 15 by 2.5 cm), sealed in air-tight polythene bags and then stored in refridgerated conditions. Prior to spectrophotometer analysis the surface layer of sediment was removed using a glass slide, and the core was then immediately covered with polyethylene film in order to avoid any water-loss and/or oxidation-induced color changes. Covering also minimizes the disruption of the sediment surface and prevents contamination of the spectrophotometer. The time elapsed between the cleaning process and the completion of the spectrophotometer analyses for each 33 cm section of core was =30 min. Measurements were generated at 1-cm intervals along the 7- m core using a Minolta CM-2022 spectrophotometer with a 4- mm diameter sample spot area. This spectrophotometer is configured to measure reflectance at 10-nm increments over the nm wavelength range. Detailed compositional information may be derived by examining features of the differential of the reflectance with respect to wavelength (see the pioneering work of Balsam and Deaton [1991]), but because our objective is to examine the nature of the variability and not to interpret the actual values, the L*a*b* output (as defined by the Commission Internationale de l'l clairage) is employed in this study. The L*a*b* scheme provides information about both the lightness and the color 311

2 _ 312 CHAPMAN AND SHACKLETON: A SPECTROPHOTOMETER STUDY a. Calibration tile only 500 N= t 300 t t.,.,.,.,., L* Calibration tile + polyethylene film 500 t N= t 300 t 200 t 100 t 01 ß i ß, :'l i. i -, L* 500 N= t N= >', (: 300 (D 300 (D 200 (D o'. '. 'o. o 'o. o' o..o'. '. ' - '. '. o%. o' o o a* b* 500 t N= t N=1050 oo:1 oo:1 ool _. _ a* b* Figure 1. Histograms of multiple replicate measurements documenting the precision of lightness (L*) and color (a* and b*) estimates. of the sediment. L* characterizes lightness (approximately equivalento grayscale reflectance) scaled from 0 (black) to 100 (white). Parameters a* and b* are chromaticity coordinates that define object color: a* measures the color shift from red (+60) to green (-60), and b* measures the color shift from yellow (+60) to blue (-60). The a'b* coordinate 0, 0 is achromatic; as a* and b* values deviate from zero, the saturation of color becomes more intense. These parameters are designed in relation to human color perception but provide a useful synthesis of the complex data collected by the spectrophotometer (we continue to use the word color for the a* and b* values). Three independent suites of measurements were generated for the core. Series 1 and 2 were analyzed along the same track in order to provide a direct assessment of analytical reproducibility. Series 3 was measured along a second track (displaced by 8-10 cm) selected so as to maximize the degree of internal variability downcore. Measurements of a white calibration tile, covered in the same polyethylene film used to cover the sediment core, were made before and after the analysis of each 33 cm-long core section in order to check for instrumental drift. The physical impediment created by the side walls of the plastic core tray made positioning the spectrophotometer flat on the sediment surface at the beginning and the end of each section difficult; as a consequence, it was necessary to discard the end measurements for some sections. 3. Results 3.1. Calibration Repeated measurement of the calibration tile demonstrates that the spectrophotometer provides a precise means of evaluating lightness and color changes (Figure 1). Standard deviations are 0.4 for L* and better than 0.1 for a* and b* (Table 1). When the calibration tile is covered with a polyethylene film identical to that used to cover the sediment core, the standard deviations of variables L* and b* approximately treble, while those of a* remain unchanged. The mean values of the three parameters L*, a*, and b* were not significantly altered by the polyethylene film (Table 1) in contrast to the observations of Curry et al. [1995] for polyvinyl chloride film. The nearly identical results obtained from analysis of different batches of polyethylene film also suggests that variability in the manufacturing process does not affect the precision of our measurements. Therefore we believe that the increase in variability observed when the calibration tile was covered by polyethylene film primarily reflects subtle differences in the positioning of the spectrophotometer measurement window on the surface of the object that is being analyzed. Visual examination and cross checking verifies that "rogue" values occur when irregularities caused by small-scale creasing and/or the presence of air bubbles trapped below the polyethylene film reduce the quality of the contact surface. However, in the vast majority of instances the physical and optical properties of the polyethylene film do not appear to bias the measurement of color or lightness. A continuous set of over 600 measurements of the calibration tile indicates that long-term instrumental drift is not a significant problem. L* and b* values are consistent throughout and show no discernible patterns over this extended series of measurements. However, there is evidence of a gradual trend toward more positive a* values which Table 1. Statistical Summary of Calibration and Downcore Spectrophotometer Measurements N L* a* b* x x y x y Calibration tile Calibration tile + polyethylene film NEAP15K series NEAP15K series NEAP15K series N, number of measurements' L*, lightness; a*, green-red ratio; b*, blue-green ratio; x, mean; and or, standard deviation.

3 CHAPMAN AND SHACKLETON: A SPECTROPHOTOMETER STUDY 313 Table 2. Reproducibility of Spectrophotometer Measurements Number of Mean Standard Deviation Data Sets L* a* b* Calibration tile + polyethylene film (sets of 10 measurements) Marine sediment + polyethylene film (sets of 5 measurements) Marine sediment + polyethylene film (sets of 10 measurements) appears to be related to the number of measurements taken since the last spectrophotometer calibration. Although the magnitude of the shift in a* values that we observed was <0.1, we recommend that the spectrophotometer should be recalibrated at least every 200 measurements in order to prevent any systematic offsets occurring in the analysis of sediment records. An additional investigation was conducted to compare the pre :ision of measurements obtained for the calibration tile and for marine sediments. This test involved making repeated measurements (sets of 5 or 10) at a single position in the core. A total of 45 sample locations were selected to represent the full extent of sediment color variability observed within core NEAP15K. The mean standard deviation of sample sets comprising 5 or 10 sediment measurements is < 0.1 for L* and better than 0.05 for a* and b* (Table 2). These results are in excellent agreement with those obtained from similar repeat measurements conducted for the polyethylene-covered calibration tile. This indicates that the high levels of precision associated with color determinations made during the calibration procedure are also attainable when analyzing marine sediment samples. Fortunately, it would also appear that the color parameters a* and b*, which typically exhibit less variability than lightness (L*), are the parameters which can be measured most precisely Downcore Measurements Considerable fluctuations exist in the downcore records of lightness and color obtained from NEAP15K (Figure 2). The presence of Ash Zone II at 636 cm in the core, dated to 52 ka in the Greenland ice core record [Gronvold et al., 1995], indicates that the NEAP15K record spans about the past 60 kyr. The range of values recorded in the downcore measurements for L*, a*, and b* are 15, 3, and 5, respectively (compared to the instrumental precision of 0.1 or better for all three spectrophotometer outputs). The differences between the lightness and color data sets measured along the same track (series 1 and 2) are negligible; correlation coefficients for L* (0.97), a* (0.99), and b* (0.92) are all significant at the 99% confidence level. Comparison of these two data sets with the measurements made along a second track (series 3) also yields correlation coefficients of 0.9 for L* and a* and 0.8 for b*. Strong similarities in the patterns of downcore variability suggesthat the effect of the minimal amount of drying and/or oxidation that could occur under the film in the time taken to make the measurements was negligible. We recognize that for the accurate characterization of the sediment that would be 60 54,,..j Figure 2. Downcore lightness and color variations in core NEAP15K. Series 1 and 2 follow the same track down the core; series 3 is from a separate track 10 cm away. In order to enable a detailed comparison, series 2 and 3 values have been systematically offset by 6 units (L*), 2 units (a*), and 4 units (b*), respectively

4 314 CHAPMAN AND SHACKLETON: A SPECTROPHOTOMETER STUDY required to assess its composition, it is desirable to dry the sediment, but this was not necessary for the objectives of this study. 4. Discussion a. 1 cm resolution o -- Series 1 -- Series 2 Series Is Downcore Variability Coherent? A visual comparison of the pattern of lightness and color signals along the different tracks reveals large-scale fluctuations at the scale of a few tens of centimeters as well as small-scale variability over depth ranges of 5 cm. Published estimates for the depth of the mixed surface sediment layer vary between 4 cm [Broecker et al., 1988] and 12 cm [Bard et al., 1987] for this region of the North Atlantic. However, the fluctuating patterns detected in the L*, a*, andb* records are often marked by sharp boundaries, even though a significant number of these features in the 20-cm-depth range comprise only a fraction of the estimated bioturbation depth. They do not exhibit the smoothed transitions one would expect to result from bioturbation processes [Ruddiman and Mcintyre, 1981]. In a few instances where perturbations due to large burrows (--1 cm wide) are well defined and can be recognized visually it is possible to verify fluctuations in the L*, a*, andb* records over a depth range of 1-4 cm. Variations over these burrowed intervals are registered by distinct changes in lightness and color that are well matched in all three downcore tracks. This suggests that it should be possible to filter out local perturbations due to burrows and construct a "cleaned" record of sediment color variability. We have chosen the L* records to examine the significance of downcore variability because high-frequency variability is more pronounced in the L* records than either a* or b*. L* measurements along the same track (L*I and L*2) are virtually identical, so these data series were averaged to provide a composite for track 1 which we compare with track 2 (L*3) located 10 cm away. Cross-spectral analysis is not appropriate for comparing centimeter-scale variability across the core because the results depend heavily on the demonstration of statistically significant peaks of variance concentration which is itself a contentious issue. Instead, we use cross correlation to examine the high-frequency component of variability. The correlation between the two L* 1.0 0, raw L*:: dat... i... i... i i Offset (cm) Offset (cm) Figure 3. Cross correlation of lightness (L*) records obtained from different downcore tracks. Track 1 represents the mean of L* 1 and L*2 values, and track 2 is L*3; the two tracks are separated by --10 cm b. 2 mm resolution Series ß, ß ß, 1.0 ß, ß )7 6;9 ''' -- Series 2... Series v 11 i ß i. i ß! Figure 4. Comparison of 1 cm- and 2 mm-resolution measurements across a 1-cm-thick horizon. L*a*b* data plotted in Figure 4a are the same as those shown in Figure 2; data shown in Figure 4b have been obtained at 2 mm intervals along three additional tracks. records as one is shifted with respect to the other shows a sharp peak at zero offset (Figure 3). A much more sensitive test is to compare differences between adjacent measurements. Cross correlation of the differenced record from track 1 and track 2 also indicates that the highest correlation occurs when both records are aligned according to their real depth in the core. Despite the very slight differences between adjacent points that are clearly largely attributable to "noise" from bioturbation, there is a degree of correlation that is entirely lost by introducing even a 1-cm offset between the two tracks (Figure 3). This demonstrates that even at the 1-cm level, sedimentary processes have not entirely obliterated the signal that might have been present in an unbioturbated record. The potential application of very high resolution studies of lightness and color variability is further exemplified by the detailed measurements made at 2 mm intervals across a distinct 1-cm thick horizon identified at a depth of 629 cm in NEAP15K (Figure 4). At present the origin of this layer has not been satisfactorily determined; increased input of biogenic silica and/or associated organic matter are considered the most likely cause. However, from the spectrophotometer records it is apparent that measurements made at 1 cm and 2 mm resolution possess a number of common characteristics. In particular, b* values show a significant increase, as do a* values to a lesser extent, but L* values decrease both in absolute terms and in terms of intradepth variability. This comparison suggests that some of the intersample fluctuations identified in the 1 cm resolution data sets may reflect real features in the sediment record rather than analytical noise Potential Resolution of Deep-Sea Records "High resolution" is a frequently occurring phrase in paleoenvironmental research, yet it is seldom possible to establish the precise resolution at which it is meaningful and

5 CHAPMAN AND SHACKLETON: A SPECTROPHOTOMETER STUDY 315 valuable to examine the geological record. The spectrophotometer is an ideal instrument to investigate this problem because it can generate millimeter-scale output that has a very high signal-to-noise ratio. Our measurements of independentracks along the 15-cm-wide sediment core show that a significant proportion of variability in the 1-2-cmdepth range may be coherent across a core. In addition to be being relatively quick the analytical procedure is nondestructive so the same core material can subsequently be used for geochemical, micropaleontological, and sedimentological studies. In fact, complimentary studies of this nature which investigate different aspects of sediment characteristics are necessary to confirm the origin of the highfrequency variability observed in the spectrophotometer records. Spectrophotometer data have been used routinely, along with other non-intrusive data sets such as magnetic susceptibility and GRAPE (gamma ray attenuation porosity evaluation) density, to establish hole-to-hole correlations at Ocean Drilling Program sites [Curry et al., 1995; Mix et al., 1992]. The extremely close match that we identify between the different high-resolution tracks in NEAP15K validates this approach and suggests that stratigraphic correlations can be made that are accurate to within a few centimeters. Preliminary investigation of spectrophotometer records obtained from cores collected from the vicinity of NEAP15K also indicate that centimeter-scale features can be used to establish highresolution correlations that are essential for detailed regional studies of climate change. 5. Conclusion Our results show that the spectrophotometer is an extraordinarily valuable tool for investigating variability in sediment sequences, often quantifying variability that cannot be observed visually. Furthermore, it is clear that the spectrophotometer is an excellent tool for addressing the question of what coherent variability exists within the geological record, and also has great stratigraphic value for establishing core-to-core correlations over wider areas. Although the maximum temporal resolution attainable within downcore color records is ultimately determined by sediment accumulation rates, the existence of abrupt changes over the 2-4-cm-depth range indicates that century-scale, and possibly even decadal-scale, fluctuations are likely to be preserved within marine sediments even after taking bioturbation processes into account. Acknowledgments. We thank S. Robinson, M. Delaney, and an anonymous reviewer for their helpful comments on the original manuscript. This research was supported by NEAPACC Special Topic grants GST/02/1822 and GST/02/1177 from the NERC. References Balsam, W.L., and B.C. Deaton, Sediment dispersal in the Atlantic Ocean: Evaluation by visible light spectra, Rev. Aquat. Sci., 4, , Bard, E., M. Arnold, P. Maurice, J. Duprat, and J.C. Dupessy, Reconstruction of the last deglaciation: Deconvolved records of õ18 0 profiles, micropaleontological variations and accelerator masspectrometric 14C dating, Clim. Dyn., 1, , Bond, G., et al., Evidence for massive discharges of icebergs into the glacial northern Atlantic, Nature, 360, , Broecker, W.S., M. Andree, W. Wolfli, H. Oeschger, G. Bonani, J. Kennett, and D. Peteet, The chronology of the last deglaciation: Implications to the cause of the Younger Dryas event, Paleoceanography, 3, 1-19, Cortijo, E., P. Yiou, L. Labeyrie, and M. Cremer, Sedimentary record of rapid climatic variability in the North Atlantic Ocean during the last glacial cycle, Paleoceanography, 10, , Curry, W.B., et al., Proceedings of the Ocean Drilling Program, Initial Reports, vol. 154, 1107 pp., Ocean Drill. Program, College Station, Tex., Dansgaard, W., et al., Evidence for general instability of past climate from a 250-kyr ice-core record, Nature, 374, , Gronvold, K., N. Oskarsson, S.J. Johnsen, H. Clausen, C.U. Hammer, G. Bond, and E. Bard, Ash layers from Iceland in the Greenland GRIP ice core correlated with oceanic and land sediments, Earth Planet. Sci. Lett., 135, , McCave, I.N., Cruise Report RRS Charles Darwin 88 (NEAPACC), 45 pp., Dep. of Earth Sci., Univ. of Cambridge, Cambridge, England, U.K., Mix, A.C., and Shipboard Scientific Party, Color reflectance spectroscopy: A tool for rapid characterization of deep-sea sediments, Proc. Ocean Drill. Program Initial Rep., 138, 67-77, Ruddiman, W.F., and A. Mcintyre, The North Atlantic Ocean during the last deglaciation, Palaeogeog. Palaeoclimatol. Palaeoecol., 35, , Taylor, K.C., G.W. Lamorey, G.A. Doyle, R.B. Alley, P.M. Grootes, P.A. Mayewski, J.W.C. White, and L.K. Barlow, The flickering switch of late Pleistocene climate change, Nature, 361, , M. R. Chapman and N.J. Shackleton, Godwin Institute for Quaternary Research, Department of Earth Sciences, University of Cambridge, New Museums Site, Pembroke Street, Cambridge CB2 3SA, England, U.K. ( (Received November 25, 1997; revised March 30, 1998; accepted March 31, 1998.)