A Revised Mid-Pliocene Composite Section Centered on the M2 Glacial Event for ODP Site 846

15 The composite section from ODP Site 846 has provided key data sets for Pliocene stable isotope and paleoclimatic time series. We document here apparent outliers in previously published data sets for stable isotopes and alkenone-derived sea surface temperature estimates (SST) in the late Pliocene interval containing the M2 glaciation (ca. 3.290-3.3 Ma) by tying high resolution core measurements to a continuous downhole conductivity log. We generate a revised sequence of 20 new stable isotopic and alkenone measurements across the M2 event that correlate well to the revised splices of color reflectance and gamma ray attenuation porosity evaluator data from Site 846, and to a new composite section produced at equatorial Pacific ODP Site 850. A new composite splice for Site 846 is proposed, along with composite isotope and alkenone time series that should be integrated into revised Pliocene paleoclimatic stacks. 25


Introduction
Continuous proxy time series from ODP Site 846 occupy a privileged place in Pliocene 35 stratigraphy and paleoclimatology. Drilled during Leg 138, this site was one of the first to systematically integrate high resolution, non-destructive core scanning into the construction of composite sections (Shipboard Scientific Party, 1992;Hagelberg et al., 1995) that formed the basis for later detailed shore-based studies. The creation of composite sections was necessitated by the fact that coring does not completely recover the sequence at any one drill hole; continuous 40 records must be stitched together to fill in coring gaps. Among the fruits of this work were some of the longest, best resolved time series of Pliocene stable isotopic variations (Shackleton et al., 1995b), lithological variations  and tropical sea surface temperatures (Lawrence et al., 2006;Herbert et al., 2010) and a proposal for an orbitally-tuned late Neogene time scale (Shackleton, 1995a). The continuity and high sedimentation rate at site 846 resulted 45 in its isotopic data set playing an outsized role in the Pliocene interval of the widely used LR04 isotopic stack (Lisiecki and Raymo, 2005. However, it was noted ( Figure 9 of Lisiecki and Raymo, 2005) that the isotopic record in the interval surrounding the glacial event M2 (circa 3.3 Ma) at Site 846 is anomalous in comparison to other sites. 50 In this contribution we document significant errors in the late Pliocene (equivalent to the Gauss magnetochron) composite section at Site 846. These affect the stable isotopic and paleotemperature reconstructions around the Pliocene M2 isotopic event, the most notable Pliocene glacial stage preceding cyclic northern hemisphere glaciation at ~ 2.7 Ma (Mudelsee and Raymo, 2005), and immediately predating the well-studied Pliocene Research, Interpretation 55 and Synoptic Mapping (PRISM) interval (Dowsett, 2007). We were drawn to revisit the original composite section because features surrounding the M2 glacial event seemed anomalous in comparison to other time-equivalent marine sections we have been investigating. We construct a more reliable composite section through a combination of tying high resolution measurements made at offset holes to a high-resolution downhole log acquired at Hole 846B, and we 60 supplement that stratigraphic analysis with new stable isotopic and alkenone sea surface temperature (SST) estimates from hole 846B, which was not used in the original composite in the M2 interval. We propose a revised composite section spanning the interval equivalent to the early Gauss through Mammoth magnetic polarity zones (circa 3.2-3.6. Ma) and tie the ODP Site 846 records to ODP Site 850, which provides an indirect tie of stable isotopic and SST estimates 65 at Site 846 to a high-quality magnetic polarity stratigraphy. Our results also yield insights into core recovery and coring distortion artifacts during hydraulic piston coring of calcareoussiliceous sediments of the equatorial Pacific. Evaluation scanner, a reflectance spectrophotometer, and magnetic susceptibility logging of cores Mix et al., 1995). Constructing a composite section proceeded by identifying tie points between holes on a core-by-core basis, splicing missing material at core breaks by the section represented at an offset hole, and assuming that a linear offset applied 80 between the drilling depth (mbsf) and the composite depth that resulted from splicing materials between holes. As compared to the original drilling depth, the composite section at Site 846 grew by about 15%, indicating that a significant amount of material was not recovered at any one hole during drilling. 85 Later sampling followed the logic of the initial splicing, hopping between holes to follow the composite section documented in the Initial Reports volume (Hagelberg et al., 1992. A more flexible and robust procedure for composite section generation was subsequently developed using dynamic programming to optimally align time series data Lisiecki, 2002, andHerbert, 2007. This method allowed for assessing distortions in the 90 coring process (e.g. stretching or squeezing of sedimentation relative to the undisturbed section) but it still required user guidance of key tie points and the more or less arbitrary assumption of which section of several replicates (e.g. holes B, C and D at Site 846) provided the least disturbed representation of sedimentation. 95 In this work we continue to use the Match program developed by Lisiecki and Lisiecki (2002), but take advantage of a high resolution downhole log of conductivity acquired at the time of drilling, which provides the best representation of a truly continuous and undistorted sedimentary column at Site 846. This opportunity was not recognized in the compositing of Lisiecki and Herbert (2007), although it could have been apparent based on the work of Harris et al. (1995), 100 who used a different hostile-environment lithodensity sonde (HLDS) log that has lower spatial resolution than the Formation Microscanner (FMS) conductivity sensor log analyzed here. The FMS log (Hole 846B pass 2) acquired data at 0.25 cm resolution from ~80 mbsf (early Pleistocene) to the base of the section (see http://brg.ldeo.columbia.edu/logdb/hole/?path=odp/leg138/846B/). Unlike many of the borehole 105 logging measurements, the conductivity log preserves fine scale (centimetric) variations related to changes in lithology and sediment physical properties. The conductivity measurements essentially represent the inverse of porosity logging, as conductivity increases with conductive pore water volume and decreases as the volume of solids grows. The conductivity log is therefore quite similar to variations in wet bulk density logged by the Gamma Ray Attenuation 110 Porosity Evaluator (GRAPE) sensor in cores raised during drilling of Site 846 (Figure 1 reports highly significant correlations of detrended data FMS and GRAPE). As demonstrated by shipboard results, wet bulk density correlates positively with carbonate content and therefore positively with reflectance (r = 0.69 for datasets from Site 846). 115 The downhole log information includes conductivity measurements from three pad sensors. We tested different combinations of weighting the log data against reflectance and GRAPE time series measured on cores (mean, geometric mean, maximum/minimum of the 3 measurements) and found that choosing the minimum of the 3 sensor measurements at each depth interval gave consistently the highest match to the core color reflectance and GRAPE data sets. We interpret 120 this result to reflect the likelihood that high conductivity readings can result from poor pad contact with the borehole and/or washouts of sediment, and are likely to be outliers. We also found that one interval of the borehole provided unreliable data from any of the sensors, most likely due to a significant washout, and deleted data from this segment (138.37 to 138.19 meters in log depth) in our alignment process. Three more short segments showed unusually low 125 conductivity and were likewise removed (146.6799-146.746, 150.3299-150.3705, and 150.4315-150.4798 meters log depth). We used core photographs to remove GRAPE and reflectance outliers at the top and base of cores based on visual evidence of core disturbance.
Mapping between offset holes and the downhole conductivity log was done to optimize the 130 alignment of GRAPE and reflectance data to the target. In addition, guided by the downhole log, we optimized the alignment of each offset hole data to the other offset holes. Data sets were normalized and detrended and we mapped ( ) reflectance and GRAPE bulk density to (-) conductivity. While we relied on the reflectance and GRAPE information, wherever possible we checked for consistency with stable isotopic (Shackleton et al., 1995b) and alkenone (Lawrence 135 et al., 2006) data sets as well. Tie points were inserted based on both reflectance and GRAPE data, but the final composites were generated from the Match algorithm applied to the GRAPE time series only, as this consistently showed better alignment to the log and smoother variations in implied core distortion than the reflectance data. The better correlations to the borelog log via GRAPE presumably reflects the benefit of consistent GRAPE calibration to density standards in 140 comparison to the reflectivity measurements and/or a closer match between GRAPE-estimated wet bulk density and conductivity, in comparison to color reflectance.
Our goal in composite section generation was to achieve alignments between the depth series of offset holes and the conductivity log robust at the scale of orbitally-related variations in 145 sedimentation and proxy variance. Given sedimentation rates in the interval of interest of 4-5 cm/kyr, this meant achieving satisfactory alignment at ~40 cm, or one half precessional wavelength. In practice, we found that approximately 20 tie points served to achieve this level of match. The Match algorithm includes a number of tradeoffs in the alignment procedure. Essentially, it determines the optimal alignment of segments of data at integer ratios, with a 1:1 150 alignment indicating no distortion of one record relative to the other. In addition to default choices supplied by the Match software (http://lorraine-lisiecki.com/match.html) we ran an experiment where we increased the number of integer choices in the vicinity of the 1:1 match in order to suppress smalls-scale jumps in relative accumulation in order to see larger patterns between offset holes (the point penalty score for correlations to the log can be improved by about 155 15% by letting the accumulation rate change more freely in the Match algorithm than we do for the "smooth" mapping). To enhance smoothness, we assigned a relatively high penalty function to the speed parameter but very little penalty to the speed change parameter-this allowed for fine scale adjustment of relative accumulation rates while minimizing large and abrupt changes in the mapping functions. We also iteratively adjusted the gaps assigned between cores from the 160 nominal gaps determined by the shipboard splices, so that the matching procedure yielded a smooth downhole mapping without artificial jumps at core breaks (Figures 2 and 3. Ultimately, we determined that choices such as the number of intervals and penalty parameters had very little effect on the final mapping (e.g. variations in parameters by a factor of 2 produced 165 negligible changes in mapping). Final Match parameter choices are presented in Table I. Figure 2 Shackleton et al. (1995b). The oxygen and carbon species-specific isotopic offsets we report are identical with (δ 18 O) or very similar to (δ 13 C) the offsets used by Shackleton et al. (1995b). 220

Alkenone unsaturation estimates
Alkenone paleothermometry relies on the temperature dependence of the degree of unsaturation 225 (number of double bonds) observed in the suite of organic compounds (C37:3 and C37:2 alkenones) synthesized by marine surface-dwelling haptophyte algae (Marlowe et al., 1984;Prahl and Wakeham, 1987). Alkenone extraction followed freeze-drying ~1 g of homogenized dry sediment, using 100% Dichloromethane (DCM) and a Dionex 200 Accelerated Solvent Extractor (ASE. Prior to quantification, extracts were evaporated with nitrogen and reconstituted with 200 230 µL of toluene spiked with n-hexatriacontane (C36) and n-heptatriacontane (C37) standards. Alkenone parameters were determined using an Agilent Technologies 6890 gas chromatographflame ionization detector (GC-FID), with Agilent Technologies DB-1 column (60 m, 0.32 mm diameter and 0.10 mm film thickness. Procedure entailed a 1µl injection, initial temperature 90 °C, increased to 255 °C with 40 °C/minute rate, increased by 1 °C/minute to 300 °C, increased 235 by 10 °C/minute to 320 °C, and an isothermal hold at 320 °C for 11 minutes. All GC analyses simultaneously provide the information to determine the U K' 37 unsaturation values and an estimate of the total amount of C37 ketones by summation of the areas of the C37:2 and C37:3 alkenones. Long-term laboratory analytical error, estimated from replicate extractions and gas chromatographic analyses of a composite sediment standard is equivalent in temperature to ±0.1 240 °C. We analyzed a total of 249 samples from hole 846B and 427 from holes 850A and 850B. All samples yielded adequate alkenone concentrations for unsaturation estimates. The mean U k' 37 value for the interval of interest from hole 846B of 0.896 compares very well to the average 245 reported by Lawrence et al. (2006)

Lessons on coring distortion 255
The Lisiecki and Herbert (2007) study of hydraulic piston coring relied on offset holes alone to assess distortion related to porosity rebound and coring-induced disturbance. This meant that distortion could be assessed from one hole relative to another, but without a definitive undistorted reference (with the exception of reported drilling depth at the top and bottom of each core). In the present case, we can assess distortions using the Match alignment of composite 260 sections to the presumably undistorted conductivity borehole log. Several conclusions stand out. First, there are very few instances of coring compression indicated by the Match algorithm (compression would register as a relative accumulation rate < 1) (Figure 3). Second, we can only document one instance of coring repetition, a segment of ~40 cm at the top of 846B core 16, consistent with the original composite section proposed by the shipboard party. 265 Lisiecki and Herbert (2007) found strong correlations of extension for Leg 138 sites with %CaCO3, and GRAPE density for the upper 50m, but decreasing correlation at depth. Our mapping of each offset hole to the downhole log shows a relationship of coring distortion between the inverse of log conductivity, composite GRAPE density, and composite reflectance 270 (not shown) over the study interval using the smoother fit option (e.g. more Match speed choices close to 1:1 between cores and the downhole log) (Figure 3). Most of the distortions correlate between offset holes (Figure 3), demonstrating the largely predictable coring expansion with lithological variations. In the case of these siliceous sediments, the coring distortion may also be related to lithology as well as physical properties, as zones with lower density (higher porosity) 275 generally have high biogenic silica contents and may behave differently from carbonatedominated lithologies during coring. While the distortion appears systematic, it somewhat counter-intuitively indicates expansion of the lower conductivity (higher carbonate/lower porosity) beds relative to lower carbonate beds when examined in detail, although the relationship is not statistically significant ( Figure 4) and may depend on unresolved factors such 280 as differences in the proportion of biogenic silica to detrital content in the non-carbonate fraction, or to the contrast of physical properties with depth. Evaluating distortion as a function of position within each core fails to reveal systematic distortion as a function of the HPC barrel penetration ( Figure 5). It therefore seems as if there is little differential distortion during the stroke of the HPC that cannot be explained by the behavior of contrasting lithologies penetrated. 285 We also found that our new composites of reflectance and GRAPE density (see Tables S2-S7) compare much more favorably to the downhole log data than the prior Lisiecki and Herbert (2007) composite. We attribute the better match both to the use of a continuous log reference and to the additional splicing constraints gained from consulting stable isotopic and alkenone 290 data. This observation highlights the importance of incorporating all available stratigraphic data to avoid mistakes in tying potentially ambiguous sections between offset holes so as to generate the most reliable composite section.

Revision of the Mammoth-Gauss/Gilbert equivalent section
To generate a revised composite, we used the downhole conductivity log as our reference. The Match algorithm allows a continuous mapping of reflectance and GRAPE data at offset Holes 846B, 846C, and 846D with log depth as the common depth scale (Tables 2-7). Data 300 obtained from any of the offset holes can be correlated to this reference depth with an uncertainty on the order of 5 cm (resolution of Match alignment). The original shipboard splice links holes 846D (the primary hole used by Shackleton et al., 1995b, and followed by Lawrence et al. 2006) and 846C. As Figure 6 demonstrates, recovery at hole 846C entirely omits the M2 interval. We turn to hole 846B, which contains the best representation of both the M2 interval and the 305 subsequent interglacial recovery, to tie its record to hole 846D. The original composite using hole 846C does nearly completely tie the gap between cores 12 and 13 of hole 846B, but is extremely stretched for the last ~2 meters by coring distortion (Figure 6). The tie lines indicated in Figure 6 include important constraints from the GRAPE log and from our new discrete δ 18 O and alkenone measurements (not shown) in addition to the reflectance data displayed. Table 8  310 provides the new composite section as a sequence of discrete segments from the different offset holes. Log depth again provides the new composite depth.
The original δ 18 O section published by Shackleton et al. (1995b) in the vicinity of the M2 glacial event seems unusual in having 2 distinctly separated enriched features (Figure 7) -an anomaly 315 that showed clearly in the original LR04 isotope stack (Figure 9 of Lisiecki and Raymo, 2005), but that nevertheless guided the final product of that stack. Carbon isotopes (not shown) also show an anomalous depletion in the same interval. Turning to a high-resolution SST record previously generated at Site 846 (Lawrence et al., 2006), it too seems anomalous, in that a strong cooling is very short lived and does not follow the reflectance, GRAPE, and stable isotopic 320 trends well in this particular interval ( Figure 9). As documented below, we are convinced that an error of unknown origin at Site 846 (erroneously labeled samples? Inverted core section?) has confused the stratigraphy surrounding the M2 event.
Both the stable isotopic and alkenone data previously generated at hole 846D deviate in an 325 anomalous manner relative to reflectance and GRAPE variations in the M2 interval (Figures 7 &  8). In general, the isotopic and alkenone values align well with variations in reflectance, log conductivity and also GRAPE bulk density at Site 846; this is the only interval we have observed with such significant deviations between proxies. In contrast, if we exclude the problematic intervals identified in Figure 9 (specifically, stable isotope data from 846D core 13, section 1, 330 and alkenone data from 846D core 13, section 2), the new composite that integrates newly generated stable isotopic and alkenone data from hole 846B follows reflectance and log data closely through the interval containing the M2 glacial event (Figure 9). It is therefore apparent that, for unknown reasons, both previously published isotopic and alkenone data that followed the original splice in the M2 interval are not reliable. We attempted several simple fixes, such as 335 assuming that stable isotope data from hole 846D, core 13, section 1, were reversed in depth by a sampling error, but the data still would not align well with the new isotope section from hole 846B, or with the reflectance and GRAPE data generated on the same section at 846D. We conclude that stable isotopic data from 846D, core 13, section 1, and alkenone data from 846D, core 13, section 2 must be discarded as erroneous. In addition, the original splice from 846D, 340 core 12 to 846C, core 12 is problematic because of what appears to be extensive coring extension at the base of core 12 at Hole 846C ( Figure 6).
We can confirm the reliability of the new isotopic and SST composites at Site 846 (see Tables S9 and S10) by comparing them to newly generated records at Site 850 ( Figure 10; Table S11). The 345 equivalent interval at Site 850 spans only one core break, minimizing possible uncertainties in creating a continuous composite section there. Unfortunately, the Pliocene section at Site 850 is too shallow to have had borehole logging, so we can only create a composite depth section based on coring (Tables S12-14). Nevertheless, isotopic and reconstructed SST patterns can be matched very precisely (sample tie lines indicated in Figure 10) between Sites 846 and 850, once 350 coring and/or sedimentation rate distortions are considered. These ties also allow us to indirectly transfer the high-quality magnetic polarity stratigraphy obtained at Site 850 to its equivalent positions at Site 846 ( Figure 10).

355
In our new composite, we align data from offset holes to the borehole conductivity log to obtain the least distorted representation of the mid to late Pliocene interval, focusing especially on the stratigraphy around the M2 interval. Careful analysis of the original composite section produced at Site 846 shows errors of unknown origin in the critical interval surrounding the M2 glacial 360 event. These errors influenced the original LR04 isotopic stack and the composite tropical ocean temperature stack of Herbert et al. (2010), and persist in the recent Ahn et al. (2017) revision to LR04, although to a lesser degree as more sites have been incorporated into the new isotopic stack. In contrast to the earlier data sets from Site 846, the M2 glaciation now stands out as a long sawtooth feature of enriched δ 18 O and cold SST values, rather than as 2 events separated by 365 a significant deglaciation/warming. The anomalously enriched interval of the δ 18 O record preceding the M2 glaciation identified as MG4 in LR04 (see Figure 9 of Lisiecki and Raymo, 2005) seems to be a 1-point outlier when new isotopic information from Hole 846B is spliced into the revised composite (Fig 8). We suggest that the new splice and composite section shown in Figure 9 replace the previously published alkenone and stable isotope sections of Lawrence et 370 al. (2006) and Shackleton et al. (1995b) and that this revised section (see Supplementary Tables  S8 and S9) be incorporated in future stable isotope and temperature stacks. Given the expanded sedimentation rate and dense sampling resolution of stable isotopic and alkenone data at Site 846, this new composite provides one of the best representations of late Pliocene paleoceanographic variability. 375

Code/Data Availability
Composite sections of non-destructive measurements (reflectance, GRAPE wet bulk density, magnetic susceptibility, borehole conductivity log from Site 846) at Sites 846 and 850 are available, with primary (hole, core, section, and centimeter depth) as well as derived (composite 380 depth) information included. New stable isotopic and alkenone determinations at Sites 846 and 850 with primary and derived (composite depth) information are also reported. All data are available as tab-delimited text files archived on the website Pangaea.

385
T. Herbert produced composite sections using the Match algorithm and wrote the bulk of the manuscript. All authors contributed to manuscript revision. R. Caballero-Gill produced alkenone SST estimates at Sites 846 and 850 and helped in the construction of composite sections. J.B. Novak picked foraminiferal samples for stable isotopic analysis and supervised running samples at the Brown Stable Isotope Facility. 390  Table 2: 846B GRAPE composite  490  Table 3: 846C GRAPE composite  Table 4: 846D GRAPE composite  Table 5: 846B reflectance composite  Table 6: 846C reflectance composite  Table 7: 846D reflectance composite  495  Table 8: 846 composite section splices Table 9: 846 composite isotopic data Table10: 846 composite SST data Table 11: 850 composite SST and isotopic data  Figure 3. Composite sections of GRAPE density generated using the Match algorithm for holes 846B, C, and D, on a new composite depth section, compared to the inverse conductivity log depth. All data sets were detrended and normalized for this comparison. Coring gaps at each hole have been filled in from offset holes to make continuous splices.  . Coring distortion inferred from Match alignment of GRAPE composite sections to the borehole conductivity log (log has been inverted, detrended, and normalized) generated using the "smooth" Match parameters emphasizing a 1:1 depth mapping of core data to the downhole log. Values >1 indicates stretching of the cored section relative to the borehole log, which we assume represents the best representation of the in situ stratigraphy. Note that the scale for coring distortion has been inverted so that higher expansion (stretching) is in the downward direction. 510 less extended more extended conductivity Figure 5. Coring distortion inferred from Match alignment of the hole 846B GRAPE composite section to the borehole conductivity log (conductivity data detrended and normalized) generated using the "high resolution" Match parameters emphasizing the closest possible mapping of core data to the downhole log. Note that core stretching generally moves positively with lower conductivity/porosity (higher carbonate content).     Shackleton et al. (1995b) and Lawrence et al. (2006), excluding the problematic intervals around the M2 glaciation (see text. The composite section has been mapped to the borehole log depth. Previously published isotopic and alkenone data in the problematic M2 interval are shown as dashed lines. Revised composite time series now align well, and the interval containing the M2 glacial event presents as a sawtooth isotopic enrichment and cooling.