Molecular distribution and isotopic composition (δD and δ13C) of lipids dataset for a sediment core (GeoB16202-2) from the north-eastern Brazilian margin

The distribution and isotopic composition (δ13C and δD) of lipids are proxies used to determinate paleoenvironmental conditions including precipitation regimes, vegetation changes and sources of organic matter, among others. This data article describes five datasets of distribution (n-alkanes, fatty acids, n-alkanols and sterols) and isotopic composition (n-alkanes and fatty acids) of lipids determined in 50 samples from a gravity core (GeoB16202–2) retrieved on the continental slope off northeastern Brazil. The core site is influenced by the Parnaiba freshwater discharge, the North Brazil Current and by the Intertropical Convergence Zone (ITCZ). Previous work focused on inorganic proxies in this core revealed important clues of climatic conditions during the Heinrich Stadial 1 (HS1). The baseline dataset of molecular and isotopic proxies of the organic matter provided here are additional and/or complimentary evidences to help elucidate past climate change during the Quaternary in the Equatorial Atlantic, where less information is available in comparison to other regions in this ocean.


a b s t r a c t
The distribution and isotopic composition ( δ 13 C and δD) of lipids are proxies used to determinate paleoenvironmental conditions including precipitation regimes, vegetation changes and sources of organic matter, among others. This data article describes five datasets of distribution ( n -alkanes, fatty acids, n -alkanols and sterols) and isotopic composition ( n -alkanes and fatty acids) of lipids determined in 50 samples from a gravity core (GeoB16202-2) retrieved on the continental slope off northeastern Brazil. The core site is influenced by the Parnaiba freshwater discharge, the North Brazil Current and by the Intertropical Convergence Zone (ITCZ). Previous work focused on inorganic proxies in this core revealed important clues of climatic conditions during the Heinrich Stadial 1 (HS1). The baseline dataset of molecular and isotopic proxies of the organic matter provided here are additional and/or complimentary evidences to help elucidate past climate change during the Quaternary in the Equatorial Atlantic, where less information is available in comparison to other regions in this ocean.  Table   Subject Environmental Sciences Specific subject area Organic Geochemistry for paleoclimate and environmental studies. Type of data Table-as supplementary material Figure  How data were acquired Analytical Instrumentation: GC-IRMS system (Gas Chromatograph model Trace Ultra coupled to mass spectrometer MAT252 from Thermo Scientific -for carbon isotope determination-and Gas Chromatograph model Trace Ultra coupled to mass spectrometer MAT253 from Thermo Scientific -for hydrogen isotope determination-) (Detection Limit 100 ng/g); Gas Chromatograph with Flame Ionization Detector GC-FID model Focus from Thermo Finnigan to determinate the distribution of n -alkanes and fatty acids (Detection Limit 2 ng/g); Gas Chromatograph coupled to mass spectrometer GC/MS model DSQ from Thermo Finnigan for n -alkanols and sterols analysis (Detection Limit 1 ng/g). Software: QGIS (version 3.10.4), Grapher (Version 15.3). Data format Raw Analyzed and calculated. Parameters for data collection Sediments samples ( n = 50) were collected from core GeoB16202-2 (773 cm) in order to achieve temporal coverage of molecular distribution and isotopic composition ( δ D and δ 13 C of lipids biomarkers) data in the continental slope off northeastern Brazil.

Description of data collection
The sediment core covers the Last Glacial Maximun (LGM), including millenial cold events as Heinrich Stadial 1 (HS1), Younger Dryas (YD) and the warm Bolling-Allerod interstadial. Lipids ( n -alkanes, n-fatty acids, n -alkanols and sterols) were extracted from sediments in an ASE (Accelerated solvent extractor), purified by column chromatography with silica and identified/quantified by GC-FID and GC/MS intrumental analyses. The isotopic composition of carbon and hydrogen of the terrigenous markers (long-chain n -alkanes and fatty acids) were determined by GC-IRMS. Selected indexes based on the lipid biomarkers -average chain lenght (ACL) and carbon preference index (CPI),-were calculated to evaluate the relative contribution of different sources of organic matter to the sediment. Data  The raw data is available as a supplementary data file to this article.

Value of the Data
• This dataset is useful to elucidate the nature and distribution of the lipids accumulated during the last 23 ka BP on the core site. This accumulation can be corelated with different climatic process and with other paleoenvironmental proxies. • This data can be used by other researchers to model climatic process that are important in this area as well to compare with other records with similar molecular and/or compoundspecific isotopic composition record. • The molecular distribution and isotopic composition data have an important role to understand the sources of organic matter accumulated in the core. The lipid biomarkers characterize autochthonous and allochthonous sources of organic matter to the study site, whereas the carbon isotope composition of lipids can indicate changes in vegetation and the hydrogen isotope composition has been shown a good proxy for hydrological regimes. Fig. 1 shows the location of sediment core GeoB16202-2 as well as the modern continental precipitation and the Parnaiba River catchment area. Tables 1-4 present, respectively, the molecular distribution of n -alkanes, fatty acids, n -alkanols and sterols, obtained in 35 samples analyzed from the sediment core GeoB16202-2. Table 5 shows the carbon and hydrogen isotopic composition of long-chain n -alkanes (C 29 , C 31 ) and long-chain fatty acids (C 26 , C 28 ) determined in 50 samples from core GeoB16202-2. Fig. 2 shows the temporal distribution of lipids ( n -alkanes, fatty acids and n -alkanols) and the variations in the values of the Carbon Preference Index (CPI) and the Average Chain Length (ACL). The CPI is a molecular ratio reflecting the relative distribution of odd-and even-carbon chain among the n -alkanes and n -fatty acids that reflects the organic matter source and/or alteration and can be related to paleoenvironmental conditions. The ACL reflects the average carbon chain length of plant leaf waxes, and has been 86  ACL 27-33 = ( n C n / C n ); CPI 25-35 = (C 25 + C 27 + C 29 + C 31 + C 33 + C 35 )/(2 (C 26 + C 28 + C 30 + C 32 + C 34 )); CPI 27-33 = ( (C 27 -33 )/ (C 26-32 )) + ( (C 27 -33 )/ (C 28 -34 )) * 0.5; Paq = (C 23 + C 25 )/(C 23 + C 25 + C 29 + C 31 ); DL = detection limit.

Table 2
Molecular distribution of n-fatty acids C 16 -C 34 in ng g −1 in the sediment core GeoB16202-2.
widely used as indicator of land vegetation and environmental conditions because woody plants has lower ACL values compared to non-woody plants. Fig. 3 shows the temporal variation of carbon isotopic composition ( δ 13 C) and hydrogen isotopic composition ( δD) for long-chain nalkanes (C 29 and C 31 ) and long-chain fatty acids (C 26 and C 28 ) and shows the change in the concentration of these compounds in the last 23 ka BP. Fig. 3. Downcore variability of isotopic composition ( δD, δ 13 C) for long-chain lipids. a) δ 13 C of n-alkanes (C 29 red, C 31 orange) and fatty acids (C 26 green, C 28 purple). b) δD of n -alkanes (C 29 blue, C 31 ligth green) and fatty acids (C 26 pink, C 28 orange). c) Total long-chain n -alkanes (C 29 red, C 31 green), and fatty acids (C 26 orange, C 28 purple).

Extraction and separation of lipids
The lipid extractions, separation and instrumental analyses (molecular distribution and isotopic composition of carbon ( δ 13 C) and hydrogen ( δD)) of n -alkanes and fatty acids were performed at MARUM-Center for Marine Environmental Science, University of Bremen. The derivatization and analyses of polar fraction were carried out at LabMAM, PUC-Rio. The age model is based on thirteen radiocarbon ( 14 C) measurements using planktonic foraminifera specimens ( G. ruber and G. sacculifer ). The results were calibrated with the IntCal13 curve (reservoir correction age of 400 ± 200 years) and the age uncertainty was modeled using the software BACON version 2.2 [7] . A total of 35 sediment samples with dry weights between 10 and 15 g were ground for lipid analysis. The extraction of lipids was carried out using a DIONEX Accelerated Solvent Extractor system (ASE 200) with a mixture of dichloromethane: methanol (9:1; v/v) as solvent at 100 °C and 1000 psi for 5 min (three cycles). A standard mixture of squalene, erucic acid and 5 α-androstanol was added to each sample prior to extraction as internal standards. After removal of elemental sulfur with copper turnings, the extracts were reduced under a N 2 flow and were subsequently saponified for 2 h at 85 °C with 1 mL of KOH (0.1 M) in methanol: H 2 O (9:1; v/v). The neutral fractions were recovered with 1 mL of hexane (3 times) and further isolated in a 1% water deactivated silica gel column chromatography (open column of 6 mm X 4 cm) into three sub-fractions ( n -alkane, ketone and polar lipids) by elution with 4 mL of hexane, dichloromethane and dichloromethane: methanol (1:1; v/v), respectively. The isolated n-alkane fraction, was further purified by elution with hexane over 4 cm AgNO 3 -coated silica column to remove unsaturated compounds. The n-alkane and ketone fractions were dried under a N 2 flow and were dissolved in 100 μL of toluene for further analysis of n -alkanes and alkenones by gas chromatography with flame ionization detector (GC-FID). The polar compounds in the neutral fraction was derivatized using 100 μL of BSTFA (N.O-bis(trimethylsilyl) trifluoroacetyl acetamide) and 250 μL of acetonitrile with heating at 80 °C for 1 h. The excess of derivatizing reagents were dried out and the residue redissolved in hexane for the determination of sterols and alkanols by gas chromatography-mass spectrometry GC/MS technique. The bulk extract residue after saponification was acidified by addition of two drops of concentrated HCl and the so-called acid fraction extracted three times with 1 mL of hexane/dichloromethane (4:1; v/v)) and then were methylated by the addition of 2 mL of acidified MeOH (with known isotopic composition of −140.8 ± 0.8 ‰ VSMOW and −55.6 ± 0.7 ‰ VPDB) with heating at 50 °C for 12 h, to yield fatty acid methyl esters (FAMEs). The FAMEs were recovered from the mixture with 3 × 1 mL of hexane and concentrated down to dryness under a gentle stream of N 2 . The dry extracts were dissolved in dichloromethane and were cleaned by elution with DCM over a column with 0.5 cm of Na 2 SO 4 and 4 cm of silica gel. The methylated fatty acids extract was transferred to 2 mL vials and was dissolved in 100 μL of toluene prior to determination by GC-FID.

Analyses of lipids by GC-FID and GC-MS
Analyses of n -alkanes and fatty acids were performed using a GC-FID system (Focus by Thermo Fisher Scientific) equipped with a capillary column Rxi-5 ms (30 m length X 0.25 mm internal diameter X 0.25 μm film coating) using helium as carrier gas (purity 99.999%) at a constant flow of 1.2 mL min −1 . The injector temperature was programmed to be constant at 260 °C and injections (1 μL) were made in splitless mode with a detector held at 280 °C. For n -alkane analyses, the GC oven temperature was set at 60 °C (held for 2 min) increased at 10 °Cmin −1 to 150 °C, and then increased at 4 °Cmin −1 to 320 °C and held for 12 min. N -alkanes were quantified by comparing peak areas of the compounds to external standard mixture (C 19 -C 34 , 10 μg mL −1 ) and to the internal squalene standard. Fatty acids were determined using a GC-oven program from 60 °C (held for 2 min), increased at 6 °C min −1 to 300 °C and held for 15 min. Fatty acids were compared with an external standard mixture (C 24 , C 26 , C 28 , C 30 ,C 32 FAMEs, 10 μg mL −1 ) and recovery was evaluated by comparing with the area of erucic acid. Precision of compound quantification is about 5% based on multiple standard analyses.
Analyses of sterols and alkanols were performed on a gas chromatography-mass spectrometry system (GC/MS Trace GC Ultra coupled to DSQ mass spectrometer, Thermo Finnigan) equipped with a DB5-ms capillary column (30 m length X 0.25 mm internal diameter X 0.25 μm film coating), using the following conditions: i) helium as carrier gas (purity 99.999%) at a constant flow of 1.2 mL min −1 ; ii) injector temperature programmed to be constant at 250 °C and injections (1μL) made in splitless mode; iii) initial oven temperature at 60 °C, temperature ramp at 20 °C min −1 up to 220 °C and then at 2 °C min −1 until 300 °C (hold for 15 min); iv) ionization source temperature at 220 °C. The GC/MS system was operated in the electron ionization (EI) mode −70 eV and Full Scan mode (monitoring the range m/z = 50-650). For quantification purposes it was used a calibration curve with certificated standards and an internal standard (5 α-cholestane). The recovery was evaluated by comparison with the signal of 5 α-androstanol added on each sample prior to extraction.

Determination of δ 13 C in lipids by GC-IRMS
Carbon isotope analyses ( δ 13 C) of individual compounds ( n -alkanes and fatty acids) were performed on a gas chromatograph Trace GC Ultra, coupled to a Finnigan MAT 252 mass spectrometer (IRMS, Thermo Scientific) via a combustion interface operated at 10 0 0 °C. Into CG-IRMS system the lipid extract is injected in splitless mode, the compounds of interest are separated by a capillary column and they are oxidized to CO 2 by a combustion reactor at 10 0 0 °C. This CO 2 produced is injected into mass spectrometer where it is analyzed. Separation of n-alkanes and fatty acids were made using a HP-5 ms capillary column (30 m length X 0.25 mm internal diameter X 0.25 μm film coating), with helium as carrier gas (purity 99.999%), at a constant flow of 1.5 mL min −1 . The injector temperature was programmed to be constant at 250 °C and the injections (2μL) were done in splitless mode. The GC temperature, for n-alkane analyses, was programmed from 120 °C (hold time 3 min), followed by heating at 5 °C min −1 to 320 °C (hold time 15 min). The oven program for fatty acids analyses was 30 °C min −1 to 170 °C, 2 °C min −1 to 215 °C (hold time of 12 min), and 20 °C min −1 to 240 °C (hold time of 15 min). The δ 13 C values for individual compounds were calibrated by injecting pulses of CO 2 as a reference gas that was automatically introduced into IRMS at the beginning and end of each analysis. The δ 13 C data is reported in per mil ( ‰ ) relative to the VPDB standard. Samples were analyzed in duplicate and the standard mixture of n -alkanes was injected every six determinations. The reported δ 13 C values represent an average of duplicates with a standard deviation less than 0.5 ‰ . The formation of fatty acid methyl ester from the fatty acid involved the addition of one methanol carbon with known isotopic composition per fatty acid molecule. The δ 13 C ratio for methanol, used to derivatize the fatty acids, was determined ( −55.6 ± 0.7 ‰ VPDB) with an elemental analyzer (EA-Flash, Thermo Scientific) to perform the correction of the isotopic compositions in methylated samples. The isotopic carbon composition of fatty acids was corrected according to published equations [ 2 , 3 ], as shown below.

Determination of δD in lipids by GC-IRMS
The δD compositions of fatty acids and n-alkanes were measured on a Thermo Trace GC, equipped with a HP-5 ms column (30 m length X 0.25 mm internal diameter X 1.0 μm film coating), coupled to a Thermo Fisher MAT 253 (IRMS) via a pyrolysis reactor (operated at 1420 °C). The GC oven program was similar to the conditions used for the analysis of the carbon isotopic composition ( δ 13 C). The measurement accuracy was controlled by the injection of standards of