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.
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, 1998b;Talarico andSandroni, 1998, 2009;Talarico et al., 2000Talarico et al., , 2012;;Sandroni 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; 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.
Seven samples, one intrusive (374-U1521A-68R-2, 58-61 cm) and six metamorphic (29R-6, 59-63 cm; 60R-5, 62-65 cm; 64R-3, 109-112 cm; 64R-6, 20-24 cm; 69R-6, 40-45 cm; and 70R-4, 120-125 cm) were selected for mineral chemistry analysis. Chemical analyses were carried out with an energy-dispersive X-ray system (Bruker Quantax 200 EDX) coupled with an electron scanning microscope (Tescan Vega3) at the Department of Physical, Earth and Environmental Sciences at the University of Siena, Italy. Analytical conditions were 20 kV accelerating voltage, 15 μA emission current, and beam spot size 0.2 μm. Natural mineral standards were used for calibration. Chemical analyses were carried out on 1 to 15 biotite crystals within each sample; crystals were selected based on the absence of alteration and/or inclusions. Cations were recalculated on the basis of 22 oxygens.
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.
Granitoid rocks are in general heterogranular and fine-to coarse-grained with a hypidiomorphic to allotriomorphic texture. Gray granitoids are more common than pinkish to reddish varieties. In some cases, they are isotropic granites (i.e., Samples 374-U1521A-35R-3, 65-72 cm; 55R-4, 2-5 cm; and 68R-2, 58-61 cm). In others, they show weakly foliated texture (i.e., Samples 29R-6, 59-63 cm; and 69R-6, 63-66 cm). Few samples show slightly porphyritic texture with orthoclase as the main porphyritic mineral (i.e., Samples 48R-6, 38-40 cm, and 55R-4, 2-5 cm). In general, femic minerals are represented by biotite, which always occurs but usually is replaced by chlorite and secondary white mica. Hornblende is rarer and often chloritized (i.e., Sample 55R-4, 2-5 cm). In some cases, white mica is directly associated with biotite (i.e., Sample 29R-6, 59-63 cm). The color index ranges from 2% to 12%. Plagioclases form subhedral to euhedral laths that are in general very altered and replaced by aggregates of saussurite (sericite ± albite ± epidote ± calcite), whereas alkali feldspars (orthoclase and/or microcline) are often replaced by kaolinite and/or sericite microaggregates. Quartz is anhedral and usually interstitial with undulose extinction in some cases (i.e., Samples 29R-6, 59-63 cm, and 69R-6, 63-66 cm). One sample (64R-3, 28-31 cm) shows mortar texture with fine-grained quartz intergrowth rimming coarser crystals. Accessory phases are in general apatite, zircon/monazite, opaque minerals, and allanite. Modal compositions range from monzogranite to granodiorite. Metamorphic rocks include metagraywackes, schists, biotite ± white mica gneisses, and phyllites. Gneiss (i.e., Sample 374-U1521A-60R-5, 62-65 cm) is heterogranular (fine to medium grained) and has granolepidoblastic texture with granoblastic domains defined by plagioclase and finegrained quartz and biotite flakes that define the rock foliation. Another gneiss (Sample 64R-6, 20-24 cm) has lepidoblastic texture defined by iso-orientation of biotite and white mica flakes and a granoblastic domain with quartz and microcline. Schists are prevalently fine to medium grained (i.e., Samples 69R-6, 40-45 cm, and 70R-4, 120-125 cm) and have weak foliation defined by orientation of biotite and rare white mica (i.e., Samples 69R-6, 40-45 cm, and 70R-4, 120-125 cm). Phyllites (i.e., Samples 62R-4, 120-125 cm, and 63R-5, 8-17 cm) show very fine grained schistosity with lepidoblastic layers defined by biotite, opaque minerals and quartz boudins (i.e., Sample 62R-4, 120-125 cm), chlorite and quartz (i.e., Sample 63R-5, 8-17 cm), and chlorite, quartz, and opaque minerals alternations (i.e., Sample 69R-1, 120-125 cm). Metagraywackes (i.e., Samples 38R-2, 134-140 cm, and 64R-3, 109-112 cm) have very fine to medium-grained heterogranular clastic texture with biotite crystals that define weak foliation; clasts are mainly composed of quartz and minorly altered feldspar grains (plagioclase is more common than alkali feldspar) set up in an abundant argillaceous matrix and sometimes calcite cement. In general, metamorphic grade of sampled clasts is low, with paragenesis typical of greenschists/subgreenschists facies. Only in a few cases (i.e., Samples 64R-6, 20-24 cm; 69R-6, 40-45 cm; and 70R-4, 120-125 cm) does mineral paragenesis point to a medium grade.
Sampled sedimentary rocks are mudstone, siltstone, sandstone, graywacke, and limestone. Siltstone (i.e., Sample 374-U1521A-63R-CC, 10-13 cm) has mainly quartzofeldspathic silt-sized grains with isotropic texture. Sandstones are laminated mature quartz arenites composed of subrounded to well-rounded quartz grains, usually with quartz overgrowth, and minor slightly altered feldspars (i.e., Sample 38R-3, 24-28 cm) and an isotropic heterogranular (medium-to very coarse grained) matrix-rich arkosic sandstone composed of monocrystalline and polycrystalline quartz and feldspar grains (i.e., Sample 71R-1, 87-90 cm). Carbonate rocks comprise heterogranular micrite to microsparites (i.e., Samples 37R-3, 110-113 cm; 66R-4, 63-67 cm; and 67R-6, 115-120 cm) and a clast-supported coarse-grained conglomerate with clasts made of sparitic limestone, oolitic limestone, heterogranular metalimestone, quartzite, and metasandstone (i.e., Sample 44R-4, 85-88 cm). Graywacke (i.e., Sample 60R-7, 90-93 cm) is composed of heterogranular (fine-to medium-grained) angular to subangular clasts made of monocrystalline quartz, minor feldspars, and felsic subvolcanic rock lithic grains.
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.
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).
(2) The second group (Samples 60R-5, 62-65 cm; 64R-6, 20-24 cm; and 68R-2, 58-61 cm) has X Fe that ranges between 0.57 and 0.66, and Al IV varies from 2.28 to 2.72 apfu.
Figure F4 shows the total number of clasts per meter along the Site U1521 core, which highlights variations in clast content and composition by lithostratigraphic unit. The number of clasts varies between 0 and 30 per meter. Within this unit, clast lithologies are dominated by granitoid rocks and metamorphic rocks (on average 30.3% and 33.9% of the total amount of counted clasts, respectively). Dolerites are widely represented (16.7%), whereas other groups are present in minor amounts: quartz fragments (9.9%), volcanic rocks (5.2%), mud intraclasts (3.7%), and sedimentary rocks (0.4%).
Lithostratigraphic Unit .17 m CSF-A) is a bioturbated diatom-bearing/rich mudstone sequence (McKay et al., 2019). This lithostratigraphic unit is characterized by the near Occurrence in percentage of each lithologic group all along the core. Percentage is calculated for each logged core; squares in the legend identify rock lithologies (modified after Marschalek et al., 2021). C. Position of sampled pebbles and cobbles and classification on the basis of thin section analysis (diamonds). See THINSECT in Supplementary material for detailed petrographic descriptions and mineral assemblage.
early Miocene ). The number of clasts is generally high, but wide variations occur along the subunit with intervals with 20 clasts per meter (i.e., Section 374-U1521A-38R-1) and intervals with as many as 160 clasts per meter (i.e., Sections 43R-2 and 43R-3). Clast lithology distribution also varies along the subunit. Granitoid rocks range from 26.3% to 45.0% of the total amount of clasts with an average value of 36.7%. Metamorphic rocks range from 36.4% to 55.2% with an average of 44.8%, and they are the most represented group. Dolerites are quite homogeneous, ranging from 5.3 to 14.2% with an average of 9.2%. Other groups are less represented (quartz fragments average 3.9%, volcanic rocks 3.3%, mud intraclasts 1.8%, and sedimentary clasts 0.3%). Six of the bulk samples from diamictites are from this unit (see 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).
Lithostratigraphic Subunit VIB (380.04-440.58 m CSF-A) consists of massive to stratified clastpoor to clast-rich diamictite (McKay et al., 2019). Generally, the number of clasts is lower than in Subunit VIA, but wide variations occur, from 20 clasts per meter (i.e., Section 374-U1521A-48R-1) to 85 clasts per meter (i.e., Section 47R-4). Subunit VIB is characterized by a decreasing downcore number of dolerites (ranging from 0.6% to 4.3% and averaging 2.8%) and by an increasing downcore number of volcanic rocks, ranging from 1.5% to 7.5% (average = 4.9%). Most of the clasts are represented by metamorphic rocks (average = 49.8%) and intrusive rocks (average = 38.3%). Other groups are scarcely represented (<5%). One sample of bulk diamictite from this unit (see Table T1) was analyzed. Generally, clast composition of this sample (see Table S1 in TABLES in Supplementary material) reflects the macroscopic classification of clasts in this unit. The metamorphic rocks group is the most represented (42%), whereas others are less represented (intrusive = 11%, basalt = 3%, felsic porphyry = 9%, and clastic sedimentary = 7%). Moreover, dolerites are nearly absent (1%) and carbonate sedimentary rocks are completely absent.
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 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).