Holocene Paleoclimate Records in Equatorial West Africa: Insights Based on the Characterization of Glycerol Dialkyl Glycerol Tetraethers

: One gravity core retrieved from the Niger Delta was used to explore the origin of deposited organic matter (OM) and the paleo-climatic and environmental conditions over the Holocene in equatorial West Africa. The geochemical properties of sediments including glycerol dialkyl glycerol tetraethers (GDGTs) and elemental (%OC, %N, C/N) and isotopic ( δ 13 C org , δ 15 N) signatures were determined. The determination constrained the age of the column and revealed that the sediment OM was mainly derived from a marine source. The isoprenoid (iso)GDGTs were the dominant GDGTs, with a small amount of branched (br)GDGTs, which led to a low-branched and isoprenoid tetraether index (BIT, 0.02–0.21) and represented a low terrestrial input. Most isoGDGTs and OH-GDGTs were produced in situ by Marine Group I (MG-I) Thaumarchaeota , while the brGDGTs were mainly transported from land. A two-endmember model quantified the contribution of terrestrial OM, as 0.9–19.9% by BIT and 1.1–32.6% by δ 13 C. Accordingly, the millennium-scale sea surface temperatures (SSTs) were reconstructed based on the cyclopentane ring distribution (TEX H86 ) and the ring index of OH-GDGTs (RI-OH). The top core SSTs were lower than the modern mean annual SST due to the growth season and habitat depth of Thaumarchaeota . The reconstructed SSTs clearly revealed the four stages of paleoclimate change, in particular, the drought episode of 8.2 kyr and the following humid period. The above research has enhanced our understanding of the paleoclimate change in river outflow during the Holocene at the millennium scale.


Introduction
Deltas and deep-sea fans represent the active interface of land and ocean and possess more than 50% of global organic carbon [1].Their sediments are optimal to investigate the pathway and fate of buried organic matter (OM) and are a valuable archive in the understanding of global carbon cycling and the reconstruction of the paleoclimate and its environment [2].
Among these areas, the Niger Delta is one of the largest deltaic systems and a major hydrocarbon-rich province, in which the river stream deposits sediments, containing various amounts of OM from West Africa through the Niger River [3].Previous research has mainly focused on oil exploration and exploitation or on thermal maturity [4].
Water 2024, 16, 771 2 of 13 Some researchers have investigated the paleoenvironmental change in the adjacent Congo Province [5].Only a few studies have assessed the sources, transportation, and fate of OM in the sediment [1].The key to understanding the paleoclimate and its environment is the reconstruction of the paleo sea surface temperature (SST).The reconstruction of the SST has mostly referred to the intra-seasonal SST with satellite data, with little reconstruction of the sediments at the millennium scale [6].The understanding of OM sources and paleo SST corresponding to the paleoclimate change in the area is still limited.
The source of OM can be traced by the bulk organic parameters, TOC, TN, the ratio of organic carbon to total nitrogen (C/N), and the isotope signatures of δ 13 C and δ 15 N [7].The typical SST proxies include Mg/Ca and δ 18 O of foraminifera [8], the U k′ 37 index based on long-chain ketones [9], and the long-chain diol index (LDI) [10].
Recently, organic biomarker proxies have been increasingly used to trace the source of sedimentary OM and to reconstruct the paleoclimatic conditions.The multiple sources may result in the overlap of C/N and stable isotope signatures of OM [11].The error and overlap could be discerned by the composition of glycerol dialkyl glycerol tetraethers (GDGTs) and the derived indicators [12].GDGTs, the reliable proxies of the paleoenvironment and paleoclimate, attract more research attention since they are preserved over geological time [13] and reveal the OM source [14], sea water temperature [15], and the structure of the microbial community [13].
The branched and isoprenoid tetraether (BIT) index [14] represents the proportion of brGDGTs to Crenarchaeol and can assess the input of terrestrial OM in a marine setting [18].The cyclopentane ring distribution of GDGTs in the surface sediment correlates with the SST, which has led to a novel proxy, TEX 86 (Equation (1) in Table 1b) [19], being used to reconstruct the SST (Equations (2) and (3) in Table 1b) [20].Modified TEX 86 , TEX H 86 and TEX L 86 , was later proposed to determine an environment with an annual SST of >15 • C and <15 • C, respectively [21].Furthermore, the ring index of OH-GDGTs (RI-OH) is suitable for the convenient reconstruction of the tropical warm SST, especially for the tropical estuarine and coastal areas [22].An error may be present in the estimation of the SST in which GDGTs are derived from both terrestrial archaea and marine methanogens [23].Fortunately, the GDGTs-related indexes can modify this kind of error.
Here, GDGT-based proxies and other organic determinations were used to investigate the source of OM and reconstruct the SST in the mouth of the Niger Delta basin.This investigation provides novel paleoenvironmental data for the African Humid Period (AHP) and a better understanding of the OM source and hydrological condition at the millennium scale.

Geological and Climate Background
The Niger Delta is located in the east of the equatorial Atlantic [24] and is bound in the south by the Gulf of Guinea (GOG) [4].The delta originated from the rifting of the Gondwana during the Late Jurassic to Neocomian [25] and the two large folds and thrust belts in the deep waters of the delta.The delta is composed of three main lithostratigraphic units, the Akata, Agbada, and Benin Formations, and the study area is located between the first two units [25].
The Niger Delta lies within the wet equatorial climatic region and is influenced mainly by the convection of the West African monsoon (WAM) [3], which regulates the moisture and heat budget of the atmosphere of low latitude and is closely related to the SST of eastern equatorial Atlantic (EEA) [24].It is a typical monsoon region with two distinct Water 2024, 16, 771 3 of 13 seasons, the summer rainy season and the winter dry season [3].The intensity and the extent of the southwest monsoon, and the main wind system in east of the GOG is related to the seasonal migration of the tropical convergence zone (ITCZ) [26].When the ITCZ approaches the equator in boreal winter, the surface temperature is the highest and vice versa (Figure 1 Left).The surface and subsurface ocean circulation there is controlled by the Guinea Current [6].In boreal summer-fall, the current is strong and brings cool and salty surface water [27], and the reverse is true in boreal winter.The Niger River drains a large part of West Africa and discharges sediment-laden water into the Atlantic Ocean [28].
Water 2024, 16, x FOR PEER REVIEW 3 of 13 SST of eastern equatorial Atlantic (EEA) [24].It is a typical monsoon region with two distinct seasons, the summer rainy season and the winter dry season [3].The intensity and the extent of the southwest monsoon, and the main wind system in east of the GOG is related to the seasonal migration of the tropical convergence zone (ITCZ) [26].When the ITCZ approaches the equator in boreal winter, the surface temperature is the highest and vice versa (Figure 1 Left).The surface and subsurface ocean circulation there is controlled by the Guinea Current [6].In boreal summer-fall, the current is strong and brings cool and salty surface water [27], and the reverse is true in boreal winter.The Niger River drains a large part of West Africa and discharges sediment-laden water into the Atlantic Ocean [28].

Sampling
One gravity core (GC)10 (3°34′15.08″N, 5°24′19.39″E) was retrieved on the northeast of the Niger Delta in February 2020 (Figure 1) during the 57IV cruise, in which the water depth was 1418 m.The core was 199 cm length and sectioned at 2 cm; in total, 67 subsamples were collected and stored at −20 °C until further analysis.

AMS 14 C Dating
Globigerinoides ruber (G.ruber) and Globigerinoides sacculifer (G.sacculifer) were chosen from seven samples and sent to Beta Analytic Inc., Miami, FL, USA, for Accelerator Mass Spectrometry (AMS) 14 C dating.Five samples contained more than 6 mg of G. ruber and G. sacculifer foraminifera shells, and the standard AMS dating was used, while the two others (depth of 142 and 199 cm) with less than 4 mg adopted Micro-sample (MS) AMS dating.The raw 14 C data were calibrated using the Marine 20 dataset [29] and expressed in calibrated years BP (years before 1950).

TOC, TN, and Stable Isotopes Analysis
The sediment was freeze-dried and ground into powder with an agate pestle.The samples were pre-treated with 6 N HCl for 24 h to remove carbonate and rinsed with

Sampling
One gravity core (GC)10 (3 • 34 ′ 15.08 ′′ N, 5 • 24 ′ 19.39 ′′ E) was retrieved on the northeast of the Niger Delta in February 2020 (Figure 1) during the 57IV cruise, in which the water depth was 1418 m.The core was 199 cm length and sectioned at 2 cm; in total, 67 sub-samples were collected and stored at −20 • C until further analysis.

AMS 14 C Dating
Globigerinoides ruber (G.ruber) and Globigerinoides sacculifer (G.sacculifer) were chosen from seven samples and sent to Beta Analytic Inc., Miami, FL, USA, for Accelerator Mass Spectrometry (AMS) 14 C dating.Five samples contained more than 6 mg of G. ruber and G. sacculifer foraminifera shells, and the standard AMS dating was used, while the two others (depth of 142 and 199 cm) with less than 4 mg adopted Micro-sample (MS) AMS dating.The raw 14 C data were calibrated using the Marine 20 dataset [29] and expressed in calibrated years BP (years before 1950).

TOC, TN, and Stable Isotopes Analysis
The sediment was freeze-dried and ground into powder with an agate pestle.The samples were pre-treated with 6 N HCl for 24 h to remove carbonate and rinsed with deionized water to remove salt.The TOC and TN were determined using a Thermo Fisher Scientific FLASH EA 1200 Series CNS at Guangdong Ocean University, with a 1σ precision of ±0.02 wt %C and ±0.003 wt %N.The TOC/TN (C/N) was also obtained.
The δ 13 C and δ 15 N were determined by a Finnigan DELTA plus XL mass spectrometer, and the determination was expressed in δ as the deviation from the standard reference material in parts per mil (‰).The reference was Vienna PeeDee Belemnite (VPDB) for δ 13 C and atmospheric nitrogen for δ 15 N.The analytical precision was ±0.2‰ for δ 13 C and ±0.3‰ for δ 15 N.

Glycerol Dialkyl Glycerol Tetraethers
The total lipid extract (TLE) of the sediment was obtained by Soxhlet extraction with DCM: MeOH (9:1 v/v) (Thermo Fisher Scientific Co., Ltd, Shanghai, China) at 47.5 • C for 72 h.The solution was filtered and transferred to a 4 mL vial.To remove the particulate matter, the TLE was redissolved in a DCM filter through a 0.45 µm PTFE filter with 500 µL 99:1 (v/v) hexane: isopropanol (ANPEL laboratory Technologies Inc., Shanghai, China).
The GDGTs were analyzed by high-performance liquid chromatography/atmospheric pressure chemical ionization-mass spectrometry (HPLC/APCI-MS) at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences.The injection was 10 µL and separated by a Prevail Cyano 3 µm column (2.1 mm 150 mm Alltech, Deerfield, IL, USA) at 30 • C. The flow rate was 0.2 mL/min.The GDGTs were isocratically eluted with 99% hexane and 1% isopropanol for 5 min, followed by a linear gradient to 1.8% isopropanol within 50 min.Detection was conducted using atmospheric pressure positive ion chemical ionization MS (APCI-MS) via selected ion monitoring (SIM) of [M + H] ions.The conditions were as follows: nebulizer pressure 60 psi, N 2 drying gas flow 6.0 L/min, gas temperature 200 • C, vaporizer temperature 400 • C, capillary voltage 2500 V, corona current 5 µA.

GDGTs Proxies and SST Estimation
The GDGT-based indices and the SST reconstruction are listed in Table 1.

Age Model
The AMS 14 C data of the core are shown in Table 2.The time scale (>8 kyr BP) covered the significant climate perturbations during the early Holocene and the more humid conditions in Africa.The sedimentation rate was high in the early-mid Holocene (~7 kyr BP, average of ~55.2 cm/kyr), and it decreased to 23.3 cm/kyr during the whole Water 2024, 16, 771 5 of 13 AHP, while it fell to ~18.8 cm/kyr after the AHP; the peak was ~8.4 kyr BP and sharply declined after 8.2 kyr BP (Figure 2a,b).The TOC decreased with depth from 1.23% to 1.87% (Figure 3).Th profile to the TOC; it changed from 0.13 to 0.19%, with an average of 0.1 changed from 10.5 to 13.The δ 13 C varied between −22.3 and −20.1‰, opposite to the δ 13 C, from 4.1 to 5.0‰.
Four stages can be defined based on the variations in the OM.The ratio were lowest in the period of ca.8.5-8 kyr BP.Then, all of these in gradually to the peak, whereas δ 13 C and δ 15 N showed bilateral symmetr BP, and the 5.5-4.7 kyr BP showed a decrease in the TOC, C/N, δ 15 N, an increase in the δ 13 C.After 4.7 kyr BP, the TOC and TN rose graduall δ 15 N showed the opposite tendency.The TOC decreased with depth from 1.23% to 1.87% (Figure 3).The TN had a similar profile to the TOC; it changed from 0.13 to 0.19%, with an average of 0.15%.The C/N ratio changed from 10.5 to 13.The δ 13 C varied between −22.3 and −20.1‰, and the δ 15 N was opposite to the δ 13 C, from 4.1 to 5.0‰.
Four stages can be defined based on the variations in the OM.The TOC, TN, and C/N ratio were lowest in the period of ca.8.5-8 kyr BP.Then, all of these indicators increased gradually to the peak, whereas δ 13 C and δ 15 N showed bilateral symmetry from 8 to 5.5 kyr BP, and the 5.5-4.7 kyr BP showed a decrease in the TOC, C/N, δ 15 N, and stable TN and an increase in the δ 13 C.After 4.7 kyr BP, the TOC and TN rose gradually, and the δ 13 C and δ 15 N showed the opposite tendency.

SST Reconstruction Based on TEX H 86 and RI-OH Indexes
The SST (Figure 5) in the Niger Delta was reconstructed by the TEX H 86 (Equations (2) and (3) in Table 1b) and the RI-OH; it was 20.7 to 26.7 °C and 20.7 to 25 °C (Equations ( 4) and ( 5) in Table 1b), respectively.The SSTs dropped at 8.4-8.2 kyr BP and slightly reduced during the humid period; then, they increased.

SST Reconstruction Based on TEX H 86 and RI-OH Indexes
The SST (Figure 5) in the Niger Delta was reconstructed by the TEX H 86 (Equations (2) and (3) in Table 1b) and the RI-OH; it was 20.7 to 26.7 °C and 20.7 to 25 °C (Equations ( 4) and ( 5) in Table 1b), respectively.The SSTs dropped at 8.4-8.2 kyr BP and slightly reduced during the humid period; then, they increased.1b) and the RI-OH; it was 20.7 to 26.7 • C and 20.7 to 25 • C (Equations ( 4) and ( 5) in Table 1b), respectively.The SSTs dropped at 8.4-8.2 kyr BP and slightly reduced during the humid period; then, they increased.

Variation in the Sources of OM during the Holocene
The proxies determined here (−22.28 to −20.08‰ for δ 13 C, 4.12 to 5‰ for δ 15 N, and 10.5 to 13 for C/N) suggested a mixture of sources of the sediment in the Niger fan.The typical δ 13 C of land C3 plants is −21 to −32‰ [34], that of C4 plants is −16 to −10‰, and that of marine phytoplankton is −18 to −21‰ [35].Terrestrial plants have a δ 15 N −5 to 18‰ [36], while the marine OM is 4-9‰ [34].The marine C/N ratio is 5-8 and >15 for land OM [35].However, the δ 15 N may be inadequate for source estimation due to the additional fractionation of nitrogen fixation and the denitrification and degradation of OM [37].The positive correlation of the TOC and TN (Supplementary Figure S1c) indicated that most nitrogen was organic [38].The BIT index (0.02 to 0.21, average 0.06, Equation (1) in Table 1a) revealed a lower contribution of land OM to the aquatic environment, since the BIT generally ranges 0.0-0.1 in a marine environment and 0.8-1.0 in land soil [39].
To further evaluate the contribution of the land OM in the delta, the two-endmember mixing model, ƒterr = (X mar − X sample)/(X mar − X terr), was applied [40], where ƒterr is the fraction of terrestrial OM, X sample can be the BIT or δ 13 C of the sample, and Xmar and Xterr are the marine and land endmembers of the BIT or δ 13 C, respectively.The calculated contribution was 0.9-19.9%(Figure 6b), adopting the equatorial Atlantic (0.03) and the African soil (0.92) for the BIT [41].For δ 13 C, the marine endmember of δ 13 C is −20‰, and the land endmember is −27‰, which use the value of C3 plants [35], which is the dominant land material in the equatorial Atlantic Ocean [42].The estimated land OM from the δ 13 C was 1.1% to 32.6%, broader than that estimated using the BIT (Figure 6b).Both estimations (Figure 6) were consistent with the determination of riverine Ba/Ca after ~5100 kyr BP [43], indicating a reduction in the land input during the late Holocene.The narrower fraction calculated by the BIT may be due to the fact that the BIT represents only the soil OM rather than the plant OM [13].This further suggested that soil OM was the main land OM delivered to the sediments (Figure 6).
During the early-mid Holocene, the TOC, TN, C/N, and BIT index were at their lowest, and the sedimentation rate rapidly declined, indicating a lower land OM input.This was caused by the reduction in the rainfall and thus in the river discharge that was ebbed

Variation in the Sources of OM during the Holocene
The proxies determined here (−22.28 to −20.08‰ for δ 13 C, 4.12 to 5‰ for δ 15 N, and 10.5 to 13 for C/N) suggested a mixture of sources of the sediment in the Niger fan.The typical δ 13 C of land C 3 plants is −21 to −32‰ [34], that of C 4 plants is −16 to −10‰, and that of marine phytoplankton is −18 to −21‰ [35].Terrestrial plants have a δ 15 N −5 to 18‰ [36], while the marine OM is 4-9‰ [34].The marine C/N ratio is 5-8 and >15 for land OM [35].However, the δ 15 N may be inadequate for source estimation due to the additional fractionation of nitrogen fixation and the denitrification and degradation of OM [37].The positive correlation of the TOC and TN (Supplementary Figure S1c) indicated that most nitrogen was organic [38].The BIT index (0.02 to 0.21, average 0.06, Equation (1) in Table 1a) revealed a lower contribution of land OM to the aquatic environment, since the BIT generally ranges 0.0-0.1 in a marine environment and 0.8-1.0 in land soil [39].
To further evaluate the contribution of the land OM in the delta, the two-endmember mixing model, ƒ terr = (X mar − X sample )/(X mar − X terr ), was applied [40], where ƒ terr is the fraction of terrestrial OM, X sample can be the BIT or δ 13 C of the sample, and X mar and X terr are the marine and land endmembers of the BIT or δ 13 C, respectively.The calculated contribution was 0.9-19.9%(Figure 6b), adopting the equatorial Atlantic (0.03) and the African soil (0.92) for the BIT [41].For δ 13 C, the marine endmember of δ 13 C is −20‰, and the land endmember is −27‰, which use the value of C 3 plants [35], which is the dominant land material in the equatorial Atlantic Ocean [42].The estimated land OM from the δ 13 C was 1.1% to 32.6%, broader than that estimated using the BIT (Figure 6b).Both estimations (Figure 6) were consistent with the determination of riverine Ba/Ca after ~5100 kyr BP [43], indicating a reduction in the land input during the late Holocene.The narrower fraction calculated by the BIT may be due to the fact that the BIT represents only the soil OM rather than the plant OM [13].This further suggested that soil OM was the main land OM delivered to the sediments (Figure 6).sity, which was induced directly by the decrease in the summer insolation and the gradual southward migration of the monsoon rain belt [5].It was also consistent with the termination of the AHP at lower latitudes [5].
The TOC increased, and the BIT decreased during 4.7~3 kyr BP.After ~3 kyr BP, the TOC was high and varied smoothly, and the BIT increased gradually, representing a humid environment [47].
The isoGDGTs, categorized further into GDGT-0, GDGT-1, GDGT-2, GDGT-3, Cren, and its regioisomer (Cren′) [19], originated mainly from Marine Group I (MG-I) Thaumarchaeota [16].GDGT-0, -1, -2, and -3 could also be produced by Marine group II (MG-II) During the early-mid Holocene, the TOC, TN, C/N, and BIT index were at their lowest, and the sedimentation rate rapidly declined, indicating a lower land OM input.This was caused by the reduction in the rainfall and thus in the river discharge that was ebbed away (Figures 3 and 6), as an interruption of the 8.2 kyr BP event and an abrupt and widespread instability of a dry and cool period in the Holocene [44].This was likely initiated by a catastrophic outflow of the proglacial Lakes Agassiz and Ojibway, with the subsequent disruption in the Atlantic meridional overturning circulation [45].The dry event during 8.5-7.8 kyr BP was also revealed by the low lake levels in Africa (i.e., Bosumtwi, Challa, Tana) [46].Around 5.5 kyr BP, the TOC, TN, and C/N increased to the peak, rapidly recovering to warm-wet during the AHP.The increase in the TOC and BIT suggested the deposition of land OM, and the decrease in the δ 13 C and the increase in the C/N reflected the high flow of fresh water.
Prevailing in ca.5.5-4.7 kyr BP, the lower input of land OM caused a subsequent decrease in the TOC and C/N (Figure 3) under post-AHP drying conditions.This timetransgressive termination of the humid period revealed the decline in the rainfall intensity, which was induced directly by the decrease in the summer insolation and the gradual southward migration of the monsoon rain belt [5].It was also consistent with the termination of the AHP at lower latitudes [5].
The TOC increased, and the BIT decreased during 4.7~3 kyr BP.After ~3 kyr BP, the TOC was high and varied smoothly, and the BIT increased gradually, representing a humid environment [47].
A small amount of land brGDGTs were revealed by the low BIT (0.02-0.21, Figure 6a), corresponding to the previous study in the Niger fan around 175 km from this study site [51].The high riverine discharge during the early Holocene was caused by increased precipitation and soil erosion, as the consequence of an intensified WAM [47].High precipitation led to the high vegetation in the Niger Basin.Furthermore, the DC (average 0.19) revealed the main land brGDGTs during the AHP and a mixture of land and marine autochthonous later (average 0.26, Figure 6a) [32].The #ringstetra changed greatly in the early Holocene and reached their peak at 5 kyr BP; then, they decreased abruptly.Specifically, they varied from 0.17 to 1.83, indicating the land-derived origin (Supplementary Table S4), which was significantly higher in the marine-derived brGDGTs (>0.7) than the land-derived brGDGTs (<0.7) [13].
Most brGDGTs were bacteria-derived and potentially sourced from land soil, similar to the case in Weijers Schouten et al., 2009.Under marine environments, particularly fans and deltas, brGDGTs can be derived from mixed marine and land sources.Three indexes, the BIT, the degree of cyclization (DC), and #rings tetra , were applied here to reveal the contribution of brGDGTs.
A small amount of land brGDGTs were revealed by the low BIT (0.02-0.21, Figure 6a), corresponding to the previous study in the Niger fan around 175 km from this study site [51].The high riverine discharge during the early Holocene was caused by increased precipitation and soil erosion, as the consequence of an intensified WAM [47].High precipitation led to the high vegetation in the Niger Basin.Furthermore, the DC (average 0.19) revealed the main land brGDGTs during the AHP and a mixture of land and marine autochthonous later (average 0.26, Figure 6a) [32].The #rings tetra changed greatly in the early Holocene and reached their peak at 5 kyr BP; then, they decreased abruptly.Specifically, they varied from 0.17 to 1.83, indicating the land-derived origin (Supplementary Table S4), which was significantly higher in the marine-derived brGDGTs (>0.7) than the land-derived brGDGTs (<0.7) [13].
The source of OH-GDGTs is complex because of the abundant diversity of archaeal communities.OH-GDGT-2 are abundant in land; the weak correlation between OH-GDGT-2 and brGDGT-Ia (Supplementary Figure S1d) indirectly implied that OH-GDGTs were probably dominated by a marine source [52].The strong correlation (Supplementary Figure S1b) between the OH-GDGTs and isoGDGTs suggested further a common source of marine Thaumarchaeota.

Reconstruction of the SST during the Holocene
The reconstruction of the TEX 86 -SST may be disturbed by the input of the land isoGDGTs [41] or other in situ marine archaea [30].As discussed above, the low BIT excluded the influence of land, and the isoGDGTs originated from Thaumarchaeota other than marine archaea.Consequently, the TEX 86 and RI-OH reconstructed reliable SSTs of the Niger fan.

Modern SSTs
The TEX 86 -SST (Figure 5) of the top layer was 23.6 • C, 4 • C lower than the current mean annual SST, determined by WOA18 data (https://www.ncei.noaa.gov/products/world-ocean-atlas, accessed on 11 October 2022), and close to the summer temperature of 30-40 m subsurface (average 23.8 • C).This layer is the predominant habitation zone of the Thaumarchaeotal community in the Guinea Basin [53].Further, the high surface temperature may cause more ammonia to fertilize Thaumarchaeota in the boreal winter in the Guinea Basin, different to other parts of Africa due to the Guinea Current [53,54], TEX H 86 may thus reconstruct a cold bias of the summer subsurface temperature.
The RI-OH-SST (21.3 • C) of the top layer was lower than the TEX 86 -SST.This low RI-OH SSTs was speculated to represent the annual temperature of the 100-200 m subsurface.The OH-GDGTs, similar to isoGDGTs, might respond differently to shallow-and deep-water communities [55], and the OH-GDGT-based indices correlated with an annual temperature of 100-200 m [33].However, high river discharge may cause a warm SST rather than the annual mean SSTs, as reported in the Chinese coastal seas [22].This deviation can be calibrated (Equation ( 6) in Table 1b); the calibrated top layer temperature was 26.8 • C, close to the modern mean annual SST (27.8 • C).Recently, a strong correlation between the OH-2/OHs and annual SST > 25 • C was reported (Table 1b, Equation ( 7)) [33]; it is suitable for this area, and the core-top SST was 27.9 • C.

Paleo SSTs
The SSTs reconstructed by the TEX H 86 and RI-OH demonstrated a slight warming since the late Holocene (Figure 5), corresponding to that of the Sanaga River mouth [24].
The GDGTs-SSTs were lower than the Mg/Ca-SST of planktonic foraminifera [24], recovered from a sediment core in the Sanaga River mouth (02 • 30.0 ′ N, 09 • 24.3 ′ E, 1328 m water depth).This difference might be caused by the differing season of growth, as Thaumarchaeotal thrive in the cool "summer", and planktonic foraminifera flourish during the warm "winter" (Figure 1 Left) [56].
Both the TEX H 86 -SST and RI-OH-SST dropped around 2 • C, corresponding to the significant dry spell in the low latitude around 8.4-8.2 kyr BP [46].Similar short cooling (~2 • C, δ 18 O-SST of G. bulloides) has also been reported in the Somalia Basin [44].A slow cooling of TEX 86 and RI-OH SST during the AHP (Figure 5) was due to the weakening monsoon.The eastern equatorial Atlantic SST was mainly controlled by the radiative forcing corresponding to the global monsoon [57].The coupling of the equatorial Atlantic SST to the WAM resulted in a low SST, weakened atmospheric convection, and reduced monsoonal precipitation.The slow rise of the SSTs after 3 kyr BP, was consistent with the freshwater records in West Africa [47].This rise was caused by the southward shift of the ITCZ and the strengthened northern trade wind [57].

Conclusions
The distribution of GDGTs, δ 13 C org , δ 15 N, TOC, and TN in a 2 m sediment core (~8.5 kyr BP) retrieved from the Niger Delta in the GOG was intensively investigated.
All the determined parameters, BIT index, and binary mixing models indicated a mixed OM source of marine and land.The BIT index, MI index, GDGT-2/Cren, and Cren/Cren ′ revealed the predominant source of GDGT to be the Thaumarchaeota group I.1 of archaea.A positive linearity of isoGDGTs and OH-GDGTs suggested that these compounds maybe produced by a common archaeal community.The BIT index, DC, and #rings tetra supported mainly land inputs of br-GDGT during the early and middle Holocene.Further, the DC revealed a mixture of land and marine autochthonous after the AHP.
Four periods were clearly revealed.In particular, the 8.2 kyr BP event was obviously demonstrated by a low TOC, TN, C/N ratio, BIT index, and a rapid decline in the sedimentation rate.A gradually decrease in land input was indicated by the decreased TOC and C/N after the humid periods (~8 to 5.5 kyr BP).
The millennium-scale SSTs were reconstructed by TEX 86 and RI-OH, and they were lower than the Holocene SSTs in the Sanaga River mouth.The habitat depth and growth season of Thaumarchaeota possibly caused the low evaluated modern SST.A precise SST was obtained by the calibration of OH-2/OHs.The paleo SSTs dropped slightly during ca.8.5-8.2 kyr BP, because of the abrupt drought in low-latitude Africa with inconspicuous cooling.
The multiple biomarkers help to shed a light on the source, fate, and dynamics of organic matter and the paleoclimate in the Niger Delta during the Holocene.

Supplementary Materials:
The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/w16050771/s1, Figure S1: Relationship between GDGTs.(a) Crenarchaeol and other iso-GDGTs; (b) iso-GDGTs and OH-GDGTs; (c) relationship between TOC and TN; (d) OH-GDGT-2 and br-GDGT-Ia; Table S1: Total organic carbon (TOC) and total nitrogen (TN) content, δ 13 C and δ 15 N values, and C/N ratio in core GC10; Table S2: Reconstruction of the paleotemperatures based on TEX 86 and RI-OH in core GC10; Table S3: Indices of br-GDGTs and percentage of terrestrial organic carbon based on a binary mixing model of δ 13 C and BIT.Table S4: Various indices were used to evaluate the source of GDGTs; Table S5: Annual average surface seawater temperature (SST) and surface seawater salinity (SSS) data of the equatorial Atlantic from 1955 to 2017 (3.5 • N; 5.5 • E).

Figure 1 .
Figure 1.Geological background of the Niger Delta.(Left): Locations of the discussed record and modern SST in the Gulf of Guinea; blue arrows indicate the Guinea Current (GC).The surface location of the Intertropical Convergence Zone (ICTZ) in northern winter (black dotted line) and northern summer (solid red line).Two green lines represent the main rivers that converge into the Gulf of Guinea (Niger River and Sanaga River).Plot modified from Ocean Data view.(Right): Niger River System and major cities; the blue dot represents the capital (modified from The Republic of Nigeria, West Africa).

Figure 1 .
Figure 1.Geological background of the Niger Delta.(Left): Locations of the discussed record and modern SST in the Gulf of Guinea; blue arrows indicate the Guinea Current (GC).The surface location of the Intertropical Convergence Zone (ICTZ) in northern winter (black dotted line) and northern summer (solid red line).Two green lines represent the main rivers that converge into the Gulf of Guinea (Niger River and Sanaga River).Plot modified from Ocean Data view.(Right): Niger River System and major cities; the blue dot represents the capital (modified from The Republic of Nigeria, West Africa).

Figure 2 .
Figure 2. (a) Age-depth model and (b) sedimentation rate of core GC10 in the

Figure 2 .
Figure 2. (a) Age-depth model and (b) sedimentation rate of core GC10 in the Niger fan.

Water 2024 , 13 Figure 3 .
Figure 3.The profile of δ 15 N, TN (total nitrogen), δ 13 C, TOC (total organic carbon), and C/N (TOC/TN).The blue bar marks the 8.2 kyr BP, and the brown bar marks the end of the humid period.The blue dashed line divides the five main stages of bulk organic parameters.

Figure 3 .
Figure 3.The profile of δ 15 N, TN (total nitrogen), δ 13 C, TOC (total organic carbon), and C/N (TOC/TN).The blue bar marks the 8.2 kyr BP, and the brown bar marks the end of the humid period.The blue dashed line divides the five main stages of bulk organic parameters.

Water 2024 , 13 Figure 3 .
Figure 3.The profile of δ 15 N, TN (total nitrogen), δ 13 C, TOC (total organic carbon), and C/N (TOC/TN).The blue bar marks the 8.2 kyr BP, and the brown bar marks the end of the humid period.The blue dashed line divides the five main stages of bulk organic parameters.

4. 4 .
SST Reconstruction Based on TEX H 86 and RI-OH IndexesThe SST (Figure5) in the Niger Delta was reconstructed by the TEX H 86 (Equations (2) and (3) in Table

Figure 5 .
Figure 5. SST (sea surface temperatures) reconstructed by various indices.The blue area indicates the surface temperature, and the brown bar marks the SST trends of the ~8.2 kry BP.

Figure 5 .
Figure 5. SST (sea surface temperatures) reconstructed by various indices.The blue area indicates the surface temperature, and the brown bar marks the SST trends of the ~8.2 kry BP.

Figure 6 .
Figure 6.(a) Profile of BIT (branched and isoprenoid tetraether index) and DC (degree of cyclization of brGDGTs) of the core GC10.(b) Percentage of soil organic carbon (%OC terr) or terrestrial organic carbon (%OC soil) based on a binary mixing model of δ 13 C and BIT.

Figure 6 .
Figure 6.(a) Profile of BIT (branched and isoprenoid tetraether index) and DC (degree of cyclization of brGDGTs) of the core GC10.(b) Percentage of soil organic carbon (%OC terr ) or terrestrial organic carbon (%OC soil ) based on a binary mixing model of δ 13 C and BIT.

Table 2 .
Details of the foraminifera AMS14C dating of the GC10 core.

Table 2 .
Details of the foraminifera AMS14C dating of the GC10 core.