Data report: petrology of gravel-sized clasts from Site U1521 core, IODP Expedition 374, Ross Sea West Antarctic Ice Sheet History

International Ocean Discovery Program (IODP) Expedition 374 recovered high-quality cores at five sites on the Ross Sea continental shelf, slope, and rise to improve the understanding of the sensitivity of the Antarctic ice sheets (and particularly the West Antarctic Ice Sheet) to past climatic and oceanic conditions, especially during a warmer-than-present climate. This report summarizes the petrology of gravel-sized clasts from Site U1521, which is located in the Pennell Basin. The recovered core spans from the early Miocene to the Pleistocene, and it is constituted by cycles of glaciomarine sediments that indicate different paleoenvironmental conditions. Granuleto cobble-sized clasts present in the sedimentary sequence have been counted and grouped into seven different lithologies based on macroscopic and microscopic recognition. The most common lithologic group is represented by low-grade metasedimentary rocks such as metasandstone, metasiltstone, and metagraywacke. Granitoid rocks (mainly monzogranite to granodiorite) are the second most represented group. Dolerites and volcanic rocks are less frequent and are abundant only in some lithostratigraphic units. Chemical analysis of biotite from seven selected metamorphic and intrusive pebbles are also provided.


Introduction
International Ocean Discovery Program (IODP) Expedition 374 (January-March 2018) recovered cores from five sites along a latitudinal and depth transect from the continental shelf to the rise in the central Ross Sea (Figure F1) to investigate the sensitivity of the West Antarctic Ice Sheet to climatic and oceanic variations in the Neogene and Quaternary (McKay et al., 2019). The principal goals of Expedition 374 were to (1) evaluate the contribution of West Antarctica to far-field ice volume and sea level estimates, (2) reconstruct ice-proximal oceanic and atmospheric temperatures to quantify past polar amplification, (3) assess the role of oceanic forcing (e.g., temperature and sea level) on Antarctic Ice Sheet (AIS) variability, (4) identify the sensitivity of the AIS to Earth's orbital configuration under a variety of climate boundary conditions, and (5) reconstruct Ross Sea paleobathymetry to examine relationships between seafloor geometry, ice sheet variability, and global climate.
Site U1521 is located in the Pennell Basin at 75°41.0351′S, 179°40.3108′W on the mid-to outer continental shelf near a northeast-southwest-oriented Miocene paleotrough ( Figure F1) (McKay et al., 2019). It consists of a single hole cored to 650.1 m drilling depth below seafloor (DSF), 411.50 m of which was recovered. Sediment is divided into seven lithostratigraphic units with different occurrences of diamictite, diatomite, diatom-rich mudstone, and mudstone as well as inter- In this data report, petrological analysis on clasts with a diameter >2 mm are presented following the division of the cores recovered at Site U1521 into seven lithostratigraphic units and the evaluation of the relative occurrence of lithologies recognized in the archive half sections.
Petrographic characterization of gravel-sized clasts provides a wide data set of the lithologic composition of these glaciomarine sediments. Lithology assemblages can be used to trace the changes in the mechanisms of gravel clast supply, which may be linked with the past dynamics of the Antarctic ice sheets.

Methods and materials
The petrology and relative abundance of clasts >2 mm were evaluated on the archive half sections of Site U1521 cores at the IODP Gulf Coast Repository at Texas A&M University (College Station, TX [USA]). Sampling, macroscopic observations, and preliminary petrographic analyses were performed following the same methods used during the Cape Roberts and Antarctic Drilling Project (ANDRILL) projects (Cape Roberts Science Team, 1998a, 1998bSandroni, 1998, 2009;Talarico et al., 2000Talarico et al., , 2012Sandroni and Talarico, 2001). Thus, each clast >2 mm (granule to cobble granulometric fraction) was macroscopically classified into one of seven broad lithologic groups: • Intrusive rocks with either isotropic and foliated varieties of felsic or intermediate granitoids and mafic intrusive rocks; • Volcanic rocks with aphyric or porphyritic, vesicular, and/or amygdule-bearing varieties ranging in composition from mafic to felsic;  • Metamorphic rocks with schistose or gneissic varieties marked by mineral orientation and various low-grade metasedimentary rocks (metalimestone, metasiltstone, metasandstone, and metagraywacke); • Sedimentary rocks with clastic lithologies (sandstone and microconglomerate with visible clastic texture) and limestone; • Intraclasts with intrabasinal sedimentary clasts, ranging from granules to cobbles, of reworked diamictite and mud clasts; • Dolerites with fine-to medium-grained holocrystalline, mafic subvolcanic rock with subophitic to ophitic texture; and • Quartz grains consisting of gray vitreous single and usually monocrystalline grains.
Data processing involved counting and classifying each clast in the different lithologic groups for every 10 cm of all Site U1521 archive half cores. A total of 15,691 clasts were counted and classified. Distribution of different lithologies was then registered for each logged core except the following: Cores 374-U1521A-1R through 4R (0-31.22 m CSF-A; Lithostratigraphic Units I and II), Cores 31R-34R (285.3-315.2 m CSF-A; Lithostratigraphic Unit V), and Core 58R (515.7-524.43 m CSF-A; Lithostratigraphic Subunit VIC). Cores 10R-22R (84.2-204.95 m CSF-A; Lithostratigraphic Unit III) were logged only for the total number of clasts without any classification because they belong to mainly diatom-bearing mudstone with rare and altered clasts (McKay et al., 2019). In addition, 26 clasts, mainly pebbles and cobbles, were sampled from the working half cores to produce standard petrographic thin sections for analysis by optical microscopy (Table T1). For each thin section, photomicrographs and a petrographic description were made (see THINSECT in Supplementary material). In addition, seven diamictite bulk samples were taken, dried, and sieved according to methodologies proposed by Perotti et al. (2018): the granulometric fraction >2 mm was mounted in epoxy and made into thin sections for petrographic analysis. The analysis aimed to identify grain lithologies present also in the bulk samples in which pebbles and cobbles are rare (see Table S1 in TABLES in Supplementary material). Classification of lithic grains followed the methods adopted in Licht et al. (2005) for metamorphic and intrusive clasts and Pompilio et al. (2007) and Panter et al. (2008) for volcanic clasts.

Clast petrography
Here we present general petrological features of logged and sampled clasts from Site U1521. For a detailed description of each thin section, see THINSECT in Supplementary material. Figure F2 shows photomicrographs of representative lithologies found in gravel-sized clasts.
Volcanic and subvolcanic rocks are represented by one sample (374-U1521A-30R-3, 85-89 cm) of altered basalt with holocrystalline, very fine grained subophitic texture made of plagioclase and clinopyroxene with minor interstitial quartz.

Mineral chemistry
A total of 73 chemical analyses were carried out on 7 clasts. Analyzed biotite crystals do not have any intracrystalline chemical variability; biotite representative compositions are shown in Table  T2. A full data set of analysis is provided in TABLES in Supplementary material. Figure F3 shows biotite composition in terms of X Fe [Fe/(Fe + Mg)] versus Al IV . In the analyzed biotites, Al IV ranges between 2.28 and 2.72 atoms per formula unit (apfu), whereas X Fe ranges between 0.40 and 0.66. In each sample, analyzed biotites usually show a small compositional range. Two main compositional groups can be identified: (1) the first has X Fe between 0.40 and 0.52 and Al IV that varies between 2.31 and 2.58 apfu (Samples 374-U1521A-29R-6, 59-63 cm; 64R-3, 109-112 cm; 69R-6, 40-45 cm; and 70R-4, 120-125 cm).

Clasts distribution
Lithostratigraphic Unit III (85.34-209.17 m CSF-A) is a bioturbated diatom-bearing/rich mudstone sequence (McKay et al., 2019). This lithostratigraphic unit is characterized by the near   absence of pebbles and cobbles; the number of gravel-size clasts ranges from 0 to 10 per meter in most of the logged cores (Cores 374-U1521A-10R through 22R) ( Figure F4A). In Lithostratigraphic Unit III, clasts were counted but not classified because they are very rare, altered, and too small to be accurately classified.  Table  T1 for sample position): percentages of counted lithic grains >2 mm from analyzed thin sections are shown in Table S1 in TABLES in Supplementary material. Overall, percentage composition of gravel-sized lithics counted in thin sections mostly reflects that of clasts logged macroscopically. Indeed, metamorphic lithics (gneisses, schists, phyllites, metasandstones, and marbles) are the dominant group, ranging from 41% to 50% in the six analyzed samples, followed by granitoid rocks (ranging from 8% to 23%) and dolerites (ranging from 10% to 23%). Sedimentary lithic fragments are present in minor numbers, but microscopic analysis allowed scientists to discriminate between clastic sedimentary lithics (sandstones, siltstones, and conglomerates, ranging from 0% to 14%) and carbonate rocks (limestones, ranging from 0% to 8% of the total amount). Basaltic volcanic lithics range from 1% to 5%, whereas felsic porphyries range from 0% to 6% (Table S1 in TABLES Supplementary material).
In Lithostratigraphic Subunit VIC (440.58-567.95 m CSF-A), which consists of interbedded clastpoor diamictite and mudstone (McKay et al., 2019), the number of clasts is generally lower than in Subunit VIA and varies between less than 10 per meter in clast-poor intervals to more than 90 per meter in clast-rich intervals. Most of the unit is characterized by clasts ranging in number from 20 to 60 per meter. Subunit VIC is similar in composition to Subunit VIB. Metamorphic rocks are most abundant (average = 50.1%), and the intrusive rock group is the second most represented (average = 37.2%). Similar to Subunit VIB, dolerite occurrence is very low (average = 2.2%). The volcanic rock group is scarcely represented (average = 2.4%), as are the other groups (<5%). Volcanic clasts do not occur continuously along the subunit; they are quite rare in the lowermost portion of the subunit (504.2-568.8 m CSF-A; Cores 374-U1521A-57R through 63R), whereas they increase in the uppermost portion of the subunit (438.9-504.2 m CSF-A; Cores 50R-57R).
Lithostratigraphic Unit VII (567.95-648.17 m CSF-A) consists of interbedded clast-poor sandy to clast-rich muddy diamictite (McKay et al., 2019) and is characterized by a generally high number of clasts, ranging from 20 per meter in clast-poor intervals up to 120 per meter in clast-rich intervals. This unit is characterized by a high abundance of metamorphic (average = 47.3%) and intrusive (average = 39.3%) rocks. In comparison with Subunits VIB and VIA, in Unit VII, dolerite amounts abruptly increase, representing, on average, 9.9% of the whole gravel fraction. The volcanic rocks group is almost absent in this unit, and the other groups are poorly represented within the clast assemblage ( Figure F4).