• The K/Pg boundary layer was preserved in marl sediments because the sedimentation occurred in low-energy water.

• The K/Pg boundary layer has elevated levels of Fe₂O₃, Ni, Co, Cr, Sc, V, Zr, Th, U, Pb, As, S, Sb, Zn, Cu, SO₂, and SO₂/MnO with a sharp decrease of calcite, CaO, and ΣREE.

• The K/Pg boundary layer is characterized by a sharp rise in redox-sensitive elements, U_EF, and Mo_EF with a dramatic drop in U/Mo ratio, indicating sulfidic-euxinic conditions that led to the development of pyrite.

The K/Pg boundary event is currently one of the greatest contentious topics in geology due to the mineralogical and geochemical evidence related to this period deposits and environmental catastrophes caused the massive extinction that characterized the Cretaceous end before ∼ 66 Ma, marking the end of the Mesozoic Era and the beginning of the Cenozoic Era. The Cretaceous period began 145 Ma ago and ended abruptly 66.04 Ma ago with the entrance of a new period of Paleogene that continued for 43 Ma (Gradstein et al., 2012; Coccioni and Silva, 2015). The geochemical and mineralogical anomalies across the K/Pg boundary at various places around the world were attributed to the impact of an extraterrestrial object with a diameter of 10 km on the Earth and/or the widespread, long-lasting activity of Deccan eruptions at the end of the Mesozoic (Alvarez et al., 1980; McLean, 1985; Hallam, 1987; Officer et al., 1987; Adatte et al., 2002). Both a large asteroid-projectile (Chicxulub) impacted Earth in the Yucatán Peninsula of Mexico (mixed target) and the Deccan Traps eruptions in India caused worldwide fires, acidic rains, and dust fogcloud that continued for a long time (Alvarez et al., 1980; Smit and Hertogen, 1980; McLean, 1985; Keller et al., 2012). These events prevented sunlight and led to a sudden global temperature drop that caused the disrubbery of the food system and mass extinction (Pope et al., 1994; Premović et al., 2004; 2006; Keller et al., 2011; Sial et al., 2019). The three events of Chicxulub impact, mass extinction, and giant pulse of Deccan eruptions possibly happened within a hundred thousand years of one another (Richards et al., 2015; Sial et al., 2019). Despite being over 13,000 km apart, the Deccan volcanism and the Chicxulub impact may have worked together to cause the end-Cretaceous mass extinction (Renne et al., 2015).

The K/Pg boundary in northern Iraq has been the subject of numerous biostratigraphical studies by various researchers, including Dunnington (1955), Al-Omari (1970), Kassab (1978), Al-Mutwali (1983), Al-Shaibani et al. (1986), Al-Qayim and Al-Shaibani (1995), Ghafor (2000), Sharbazheri et al. (2011), Al-Sheikhly et al. (2015), Lawa (2018), Al-Qayim et al. (2020), and Bamerni (2021; 2022). However, Salih et al. (2013) were used geochemical data to determine the boundary between the Shiranish and Kolosh formations in the Duhok Dam area of northern Iraq. The current paper provides mineralogical and geochemical evidence to detect and confirm the K/Pg boundary in the Bade section of northern Iraq, which has been proposed by the biostratigraphical study of Bamerni (2021; 2022).

The study area (Bade section) is located in Dohuk governorate of northern Iraq, roughly 10 km northeast of Dohuk city, 7 km northeast of Dohuk Dam Lake, and 1.2 km southwest of Bade village (Fig. 1). At the exact junction of coordinates of latitude 36° 53′ 52.95′′ N and longitude 43° 05′ 5.16′′ E, the Bade section is exposed on the Bekhair anticlinés northern limb (Fig. 1). Tectonically, the Bade section is situated within the High Folded Zone of the Unstable Shelf according to the tectonic divisions of Iraq by Buday and Jassim (1987) (Fig. 2).

Figure 1. Location of the study area. (A) Map of Iraq shows the location of Bade section at Dohuk governorate. (B) Aerial photo of a specific area within the Dohuk governorate shows the Bade section is located about 1.2 km southwest of Bade village and 10 km northeast of Dohuk city.

Figure 2. Tectonic division map of Iraq (modified from Buday and Jassim, 1984) identifies the tectonic zones of Unstable and Stable shelves. The Unstable Shelf is divided into Foothill Zone, High Folded Zone, Imbricated Zone, and Zagros Suture Zone. On the other hand, the Stable Shelf is divided into Rutba Jezira Zone, Salman Zone, and Mesopotamian Zone. This map shows the studied Bade section is located within the High Folded Zone of the Unstable Shelf.

Generally, Iraq is located in the northeastern part of the Arabian Plate (Husseini, 1992). Northern Iraq is part of the Alpine Orogenic Zone, formed during the Lower Cretaceous collision between the Afro-Arabian and Eurasian plates (Sharland et al., 2001; Numan, 2001). A foreland basin succession was formed on the Arabian Platés northern passive margin due to crust loading by thrust sheets caused by the Iranian Plate compressions on its northeastern margin (Jassim and Goff, 2006). During the Campanian-Maastrichtian ages, ophiolites and radiolarites were obducted, eroded, and re-deposited as Tanjero Formation's flysch in the foredeep basin along the eastern and northern margins of the Arabian plate (Sharland et al., 2001; Ahmed, 2021; Al-Hamidi et al., 2023). The Tanjero and Shiranish formations were deposited in the narrow foreland basin in northern Iraq, where the Tanjero Formation united tectonically with the underlying Shiranish Formation to form just one basin, commonly known as the earliest Zagros foreland basin (Karim and Surdashy, 2005; Ahmed, 2021).

The Paleocene-Eocene epochs marked the closure of the Neo-Tethys Ocean, as the convergence of Eurasia and Afro-Arabian plates was primarily consumed in the subduction zone under the southern Eurasia Plate (Jassim and Goff, 2006; Saura et al., 2011; Ahmed, 2021). During these epochs, new lands and mountain ranges emerged in the northeastern Arabian Plate, serving as the primary source of clastic sediments for the Kolosh Formation during the Paleocene and Gercus Formation during the Paleocene- Eocene (Jassim and Goff, 2006; Ahmed, 2021). In the High Folded Zone, the reefal carbonate facies of the Khurmala, Aaliji, and Sinjar formations were deposited alongside the Kolosh Formation during the Upper Paleocene-Lower Eocene, while the Avanah and Pila Spi formations were deposited in a shallow lagoon during the Middle-Upper Eocene (Jassim and Goff, 2006; Aqrawi et al., 2010; Ahmed, 2021). The Neo-Tethys was likely closed at the end of the Upper Eocene (Jassim and Goff, 2006). The Oligocene saw the completion of the Arabia-Central Iran suture along the Main Zagros Thrust-Belt (Stoneley, 1981).

3.1. Fieldwork and Sampling

The Bade section represents the exposures of the Shiranish and Aaliji formations across the K/Pg boundary. The biostratigraphical study of Bamerni (2021; 2022) considered the first defined boundary between Shiranish and Aaliji formations in the Bade section. However, fieldwork revealed that the total thickness of the Bade section is about 48 m, represented by 25 m of the Shiranish Formation and 23 m of the Aaliji Formation (Fig. 3).

Figure 3. Lithostratigraphic column across the K/Pg boundary for the Bade section, Dohuk area, northern Iraq. Arrows indicate the magnification of the Cretaceous-Paleogene transition.

The lower part of the Shiranish Formation conformably and abruptly overlies the dark brown dolomitized limestone of the Bekhme Formation. On the other hand, the upper part of the Shiranish Formation ends with 3 m of bluish-gray marl crusted by black spherules that appear to be iron oxide with diameters between 0.1 and 0.4 mm found in the matrix or groundmass of iron sediment with a thickness of only 0.5 cm (Fig. 4). This iron layer of spherules and matrix (iron spherule-rich layer) has been proposed as the K/Pg boundary between two different marl colors: bluishgray for the upper part of the Shiranish Formation and yellowish-brown for the lower part of the Aaliji Formation (Figs. 3, 4). The upper part of the Aaliji Formation ends with about 50-60 cm of light brown hard marly limestone; this is the ultimate bed beneath the Kolosh Formation in the study area. Based on changes in lithological characteristics and bed colors, thirty-nine samples were collected with sampling distances intensified to vary between 0.5 and 5 cm at the field-observed boundary (possibly K/Pg).

Figure 4. Lithological contact between Shiranish and Aaliji formations of the studied Bade section, Dohuk area, northern Iraq, showing two different marl colors (bluish-gray for Shiranish and yellowish-brown for Aaliji) and an iron spherule-rich layer (sample B20) composed of black iron spherules found in an iron matrix.

3.2. Analytical Methods

This work's analytical methodologies included determining insoluble residues (IR) to eliminate the vast majority of the calcite and preparing thin sections for petrographic study, as well as analysis using X-ray diffraction (XRD), fusion X-ray fluorescence (XRF), and inductively coupled plasmamass spectrometry (ICP-MS). For selected samples across the K/Pg boundary, the IR method and petrographic study were conducted in the geochemical laboratory, Geology Department, College of Science, University of Mosul. At the same time, XRD, fusion XRF, and ICP-MS techniques were carried out in the laboratories of the Iranian University of Science and Technology (IUST), Tehran.

Six sample powders (B19, B19 IR, B20, B21, B21 IR, and B22 IR) were analyzed using the XRD technique to determine the mineralogical composition. The XRD instrument type used for analysis is the PW3830 X-ray generator under conditions of 40 KV voltage, 20 mA current, Cu Kα Cu-tube, 1 cm/minute speed of chart, 2θ degree/minute speed of goniometer, and a range of 2θ recording from 4° to 60°. Moreover, the type of fusion XRF instrument used to analyze the major oxides and loss on ignition (LOI) for five samples (B18, B19, B20, B21, and B22) is the Model PW1480 sequential spectrometer, which generates X-rays under conditions of 60 KV, 40 mA, and chromium target. Trace and rare earth elements for seven powders of samples (B17, B18, B19, B20, B21, B22, and B23) and major oxides of B17 and B23 sample powders were analyzed using the Agilent 7700 series ICP-MS instrument, where 0.25 g of sample powder was dried at 60°C and digested with 4- acid solution mixture (H₂O₂, HF, HClO₄, HNO₃) by adding 10 ml of this mixture to the sample powder in ratios (2:2:1:1), respectively, to obtain a solution ready for analysis by the ICP-MS technique.

4.1. Petrography

The petrographic study shows that the upper part of the Shiranish Formation (sample B19) consists of 50-60% globular planktonic foraminifera (Fig. 5 A, B, C) with about 5% extraclast of polycrystalline quartz (Fig. 5 D). Calcite occurs in a matrix and fills the chambers of fossil skeletons. These components are embedded in a fine micritic matrix consisting of microcrystalline calcite crystals that are less than 4 μm in size. A fine micritic matrix indicates that the sedimentation occurred in calm water free of removal currents (Folk, 1974). According to Dunham (1962), the micrite and proportion of skeletal grains suggest fossiliferous lime packstone microfacies (Fig. 5 A, B, C). On the other hand, the proposed K/Pg boundary (sample B20) consists wholly of iron oxide (Fig. 5 E).

Figure 5. Photomicrographs of the studied Bade section samples. (A) and (B) Fossiliferous lime packstone microfacies show globular planktonic foraminifera (sample B19). (C) Globular planktonic foraminifera and phosphatic grain with cementation (sample B19). (D) Polycrystalline quartz (sample B19). (E) Iron oxide from the iron spherule-rich layer (K/Pg; sample B20). (F) and (G) Fossiliferous lime mudstone microfacies (sample B21). (H) and (I) Fossiliferous bioclast lime grainstone microfacies (sample B39). Magnification = 5X for A and H; 40X for B, C, D, E, F, G, and I.

The lower part of the Aaliji Formation (sample B21) comprises mud-supported skeletal grains (< 10%) and micritic matrix proportions up to 90%. According to Dunham (1962), these proportions of mud-supported grains and micritic matrix indicate fossiliferous lime mudstone microfacies (Fig. 5 F, G), which are low-energy sediments (Flügel, 2010). Moreover, the upper part of the Aaliji Formation (sample B39) consists of 70-80% bioclasts embedded in a microspar matrix (Fig. 5 H, I). Dunham (1962) proposes that this bioclast proportion indicates fossiliferous bioclast lime grainstone microfacies (Fig. 5 F, G), which are indicative of high-energy environmental sediments (Flügel, 2010).

The petrographic study of the Bade section revealed that sedimentation just below and above the proposed K/Pg boundary occurred in calm, low-water energy, which may have helped preserve the iron spherule-rich layer (K/Pg) in the sediments. Furthermore, the abundance of planktonic foraminifera assemblages directly below the K/Pg boundary (sample B19), their disappearance at the boundary layer (sample B20), and their slight appearance just after the boundary (sample B21) suggest changes in environmental conditions due to the Chicxulub and/or Deccan events (Smit and Romein, 1985; Smit, 1982; Pope et al., 1994; Arenillas et al., 2004; Keller et al., 2011; Alegret et al., 2012; Bardeen et al., 2017).

4.2. Mineralogy

The mineralogical composition of the Upper Cretaceous (sample B19) is comprised primarily of calcite, quartz, and muscovite phases (Fig. 6 A, B; Table 1). On the other hand, the proposed K/Pg boundary layer (sample B20 “iron spherule-rich layer”) consists mainly of goethite with a minor amount of barite; hence, it will be referred to as the goethite layer (Fig. 6 C; Table 1). Moreover, the mineralogical composition of Lower Paleogene (samples B21 and B22) is composed of calcite, quartz, muscovite, and anatase phases (Fig. 7 A, B, C; Table 1).

Table 1 . Semi-quantitative mineralogical proportions across the K/Pg boundary for the Bade section, Dohuk area, northern Iraq. IR refers to the samples that were processed by the insoluble residues method.

Figure 6. XRD pattern of the studied Bade section samples. (A) and (B) Upper Cretaceous layer (sample B19). (C) K/Pg boundary layer (sample B20). IR refers to the samples that were processed by the insoluble residues method.

Figure 7. XRD pattern of the studied Bade section samples. (A), (B), and (C) Lower Paleogene layers (samples B21 and B22). IR refers to the samples that were processed by the insoluble residues method.

Calcite decreases sharply at the goethite layer (sample B20) and increases significantly at the Lower Paleogene layer (sample B21) (Table 1). A sharp decrease in calcite could be linked to changes in conditions caused by Chicxulub and/or Deccan events, such as acidic rain, sunlight prevention, and a sudden global temperature drop. These conditions may have temporarily halted the development of biogenic calcite by dissolving or inhibiting planktonic carbonate (Pope et al., 1994; Premović et al., 2004; 2006; Barash, 2011; Keller et al., 2011).

According to XRD results (Fig. 6 C; Table 1), the matrix and spherules of the proposed K/Pg boundary (sample B20) are composed of the goethite phase. Goethite spherules are common materials in the globally K/Pg boundary deposits (Kyte et al., 1980; Gilmour and Anders, 1989; Schmitz, 1992). Generally, three spherule forms with different compositions can be identified in the distal K/Pg deposits: black spherules of goethite, yellow-whitish spherules of k-feldspar, and glauconitic-composed green spherules, where the components of all three kinds are diagenetic substitution products of earlier, unidentified compositions (Belza et al., 2017). Goethite may be developed from olivine (iddingsite) or pyrite compositions released by Chicxulub and/or Deccan activities (Arakawa et al., 2003; Belza et al., 2017). Moreover, the presence of barite in the proposed K/Pg boundary layer (goethite layer) (Fig. 6 C; Table 1) may be attributed to an early diagenetic stage or a descended sulfate aerosol of the Chicxulub impact and/or Deccan volcanism that reacted with sea barium to form barite.

The K/Pg boundary of the studied Bade section is identical to all other globally well-known K/Pg boundaries, such as Stevns Klint in Denmark, El Kef in Tunisia, Agost and Caravaca in Spain, and Blake Nose in the NW Atlantic. These boundaries are distinguished mineralogically by goethite spherules and a sudden drop in calcite content. Hansen et al. (1986) claimed goethite-dominated spherules in the Stevns Klint boundary, while those from the Caravaca boundary are made up of goethite and sanidine. At El Kef and Agost, the boundaries consist of K-feldspar and goethite spherules with a sharp decrease in calcite (Montanari 1991; Ruiz et al., 1992, 1997; Molina et al., 2005, 2006). According to Premović et al. (2004), goethite makes up most of the Feoxide spherules at the Blake Nose boundary, which is characterized by a sharp drop in calcite.

4.3. Geochemistry

The chemical compositions of major, trace, and rare earth elements are given in Table 2. The Pearson correlation of elements was calculated using SPSS Statistics 19.0 software. Major oxides vary based on the CaO content. CaO shows a positive correlation with LOI (r = 0.99) and negative correlations with SiO₂, P₂O₅, MgO, and K₂O (r ^CaO:SiO2 = -0.96, r ^CaO:P2O5 = -0.99, r ^CaO:MgO = -0.80, and r ^CaO:K₂O = -0.64). These indicate the correlations of CaO with calcite and SiO₂, P₂O₅, MgO, and K₂O with non-carbonate phases. Transition trace elements (TTEs) of Ni, Co, Cr, Sc, and V exhibit positive correlations with Fe₂O₃ (r = 0.82-0.99), suggesting they are related to the goethite layer. A positive correlation of Zr with Fe₂O₃ (r = 0.54) and TTEs (r = 0.52-0.69) indicates that it is associated with the goethite phase. Moreover, the positive correlations of Hf and Ta with SiO₂, Al₂O₃, Fe₂O₃, K₂O, TiO₂, and MnO (r ^Hf:SiO2 = 0.57, r ^Ta:SiO2 = 0.53, r ^Hf:Al2O3 = 0.61, r ^Ta:Al2O3 = 0.55, r ^Hf:Fe2O3 = 0.77, r ^Ta:Fe2O3= 0.93, r ^Hf:K2O = 0.68, r ^Ta:K2O = 0.93, r ^Hf:TiO2 = 0.86, r ^Ta:TiO2= 0.90, r ^Hf:MnO = 0.64, and r ^Ta:MnO = 0.72) reflect that they are incorporated into white mica (muscovite), goethite, and anatase phases. Ba exhibits positive correlations with Fe₂O₃ (r = 0.99), TTEs (r = 0.74-0.99), Zr (r = 0.57), and S (r = 0.58), indicating that Ba is associated with goethite and barite phases. Rb, Sr, Cs, and Li elements are associated with calcite and muscovite minerals through positive correlations with LOI (loss on ignition) and all major oxides except Fe₂O₃. Furthermore, Pb is associated with goethite and barite phases due to its positive correlations with Fe₂O₃ (r = 0.98), TTEs (r = 0.75-0.99), Zr (r = 0.62), Ba (r = 0.98), and S (r = 0.59) and its negative correlations with SiO₂, Al₂O₃, MgO, CaO, Na₂O, K₂O, TiO₂, P₂O₅, MnO, LOI, Rb, Sr, Cs, and Li. As, Cd, In, Mo, S, Sb, Te, W, Cu, and Zn e lements a re a ssociated with goethite because they show positive correlations with Fe₂O₃ (r = 0.50- 0.99) and negative correlations with other oxides.

Table 2 . Concentrations of major (in wt%), trace (in ppm), and rare earth elements (in ppm) across the K/Pg boundary for the Bade section, Dohuk area, northern Iraq. Major oxides are analyzed by XRF; trace and rare earth elements are analyzed by ICP-MS; * = major oxides are analyzed by ICP-MS; LOI = loss on ignition; n = chondrite-normalized REE values are taken from Sun and McDonough (1989); Eu/Eu* = (Eu)n/[(Sm)n × (Gd)n]^0.5 is from Kato et al. (2006); Ce/Ce* = (Ce)n/[(La)n × (Pr)n]^0.5 is from Alfaro et al. (2018).

Elements	B23	B22	B21	B20 (K/Pg)	B19	B18	B17	Min.	Max.	Average
SiO2		14.09	16.48	6.08	15.69	17.68		6.08	17.68	14.00
Al2O3	2.68*	3.16	3.60	1.31	4.21	3.31	2.49*	1.31	4.21	2.97
Fe2O3	0.87*	2.57	3.45	75.03	2.79	2.40	2.03*	0.87	75.03	12.73
MgO	2.22*	2.19	2.78	0.77	2.46	3.70	2.26*	0.77	3.70	2.34
CaO	>5.0*	41.8	38.71	1.96	39.63	38.39	>5.0*	1.96	41.80	32.10
Na2O	0.07*	0.09	0.11	0.01	0.220	0.15	0.08*	0.01	0.22	0.10
K2O	0.61*	0.58	0.69	0.20	0.610	0.61	0.53*	0.20	0.69	0.55
TiO2	0.20*	0.247	0.284	0.109	0.259	0.233	0.17*	0.11	0.28	0.21
P2O5	0.18*	0.155	0.281	0.189	0.252	0.311	0.18*	0.16	0.31	0.22
MnO	0.03*	0.035	0.041	0.021	0.033	0.031	0.03*	0.02	0.04	0.03
LOI		34.98	33.30	14.08	33.55	33.07		14.08	34.98	29.80
Total		99.90	99.73	99.76	99.70	99.89
Ni	129	130	200	1347	151	187	121	121	1347	323.57
Co	15.23	13.85	19.83	83.31	12.82	18.98	12.48	12.48	83.31	25.21
Cr	55	63	94	315	70	97	69	55	315	109
Sc	6.8	7.6	9.3	10.6	8.1	8.4	6.3	6.3	10.6	8.16
V	46	56	74	510	55	57	51	46	510	121.29
Zr	15	<1	9	17	<1	11	<1	<1	17	13
Hf	10.25	11.36	11.25	8.41	9.58	11.36	10.25	8.41	11.36	10.35
Nb	7.95	4.69	1.85	3.25	4.88	6.14	4.25	1.85	7.95	4.72
Ta	3.24	5.65	4.58	3.69	3.21	4.25	3.25	3.21	5.65	3.98
Th	12.58	13.25	18.25	10.78	6.58	9.58	10.24	6.58	18.25	11.61
U	3.52	4.52	1.85	3.25	2.25	3.65	4.25	1.85	4.52	3.33
Y	13.65	13.25	17.25	11.85	12.25	14.52	13.25	11.85	17.25	13.72
Ba	42.1	34.6	52.2	3185.6	56.2	87.3	35.2	34.6	3185.6	499.03
Rb	12.36	19.25	12.52	11.36	17.36	14.25	16.21	11.36	19.25	14.76
Sr	654.1	707.3	691.2	214	667	666	677.9	214	707.3	611.07
Cs	6.25	7.25	9.41	6.25	8.58	7.25	6.52	6.25	9.41	7.36
Li	15	16	21	5	20	22	16	5	22	16.43
Ga	2.52	6.36	4.54	2.78	1.62	3.25	2.14	1.62	6.36	3.32
Pb	1.02	1.12	1.58	121.31	1.25	25.12	5.21	1.02	121.31	22.37
Ag	0.12	0.62	0.14	0.31	0.45	0.21	0.12	0.12	0.62	0.28
As	2	6	2	64	3	4	2	2	64	11.86
Be	0.62	0.25	0.74	0.26	0.58	0.41	0.25	0.25	0.74	0.44
Bi	6.41	6.21	1.25	8.28	2.14	8.41	2.21	1.25	8.41	4.99
Cd	0.31	0.25	0.14	3.01	0.45	0.32	0.21	0.14	3.01	0.67
In	0.36	0.14	0.25	0.62	0.47	0.25	0.89	0.14	0.89	0.43
Mo	1.98	2.49	3.62	53.85	2.92	2.29	2.14	1.98	53.85	9.90
S	1300	400	300	1300	300	1600	700	300	1600	842.86
Sb	1.25	2.14	7.12	26.01	3.56	1.25	2.11	1.25	26.01	6.21
Sn	8.67	10.09	5.4	10.96	11.46	9.52	11.1	5.4	11.46	9.60
Te	0.51	0.26	0.47	0.85	0.36	0.25	0.13	0.13	0.85	0.40
Tl	3.25	4.12	3.25	0.69	2.25	0.95	1.21	0.69	4.12	2.25
W	4.89	7.25	3.25	47.12	3.25	1.47	2.54	1.47	47.12	9.97
Cu	15.2	16.4	22.3	271.6	22.1	22.3	18.2	15.2	271.6	55.44
Zn	77.1	92.5	143.6	779.8	46.6	46.6	59.1	46.6	779.8	177.90
La	8.30	8.95	10.70	4.86	11.06	9.73	9.35	4.86	11.06	8.99
Ce	17.32	19.25	20.36	10.36	23.65	21.56	20.36	10.36	23.65	18.98
Pr	3.95	3.58	4.59	2.80	6.21	3.27	3.25	2.80	6.21	3.95
Nd	6.89	7.21	8.69	3.69	10.25	7.21	7.89	3.69	10.25	7.40
Sm	2.69	3.36	5.58	2.32	7.21	3.99	4.21	2.32	7.21	4.19
Eu	1.59	2.06	3.25	1.20	4.25	2.12	1.25	1.20	4.25	2.25
Gd	2.56	3.12	5.01	2.06	6.21	3.59	3.56	2.06	6.21	3.73
Tb	1.23	1.59	2.79	0.89	4.01	1.78	1.03	0.89	4.01	1.90
Dy	2.03	2.89	4.21	1.58	5.49	3.12	2.58	1.58	5.49	3.13
Ho	1.12	1.25	2.03	0.69	3.21	1.21	0.89	0.69	3.21	1.49
Er	1.78	2.26	3.56	1.26	5.03	2.89	2.13	1.26	5.03	2.70
Tm	0.63	1.03	1.69	0.49	2.42	0.78	0.49	0.49	2.42	1.08
Yb	1.26	1.98	3.01	0.96	4.28	2.48	1.45	0.96	4.28	2.20
Lu	0.39	0.69	1.06	0.20	2.03	0.32	0.19	0.19	2.03	0.70
LREE	40.74	44.41	53.17	25.23	62.63	47.88	46.31	25.23	62.63	45.77
HREE	11.00	14.81	23.36	8.13	32.68	16.17	12.32	8.13	32.68	16.92
LREE/HREE	3.70	3.00	2.28	3.10	1.92	2.96	3.76	3.10	1.92	2.70
ΣREE	51.74	59.22	76.53	33.36	95.31	64.05	58.63	33.36	95.31	62.69
(La/Sm)n	1.99	1.72	1.24	1.35	0.99	1.57	1.43	0.99	1.99	1.47
(Gd/Yb)n	1.68	1.30	1.37	1.77	1.20	1.19	2.03	1.19	2.03	1.51
(La/Yb)n	4.73	3.24	2.55	3.63	1.85	2.81	4.63	1.85	4.73	3.35
Eu/Eu*	1.85	1.95	1.88	1.68	1.94	1.71	0.99	0.99	1.95	1.72
Ce/Ce*	0.74	0.83	0.71	0.69	0.70	0.94	0.91	0.69	0.94	0.79

Chondrite-normalized REE patterns (Fig. 8) exhibit a one-package zigzag pattern, which could indicate a shared provenance for the studied sediments. Generally, the REE pattern differs from Upper Continental Crust (UCC) and Post-Archean Australian Shale (PAAS) patterns and displays more enrichment than chondrite. The pattern shows a slight enrichment of light rare earth elements (LREEs) relative to heavy rare earth elements (HREEs) and elevated Tb and Tm (Table 2; Fig. 8) due to the REE scavenging mechanism from seawater (De Baar et al., 1985). Additionally, most sample sediments show positive Eu and slightly negative Ce anomalies (Table 2; Fig. 8). The positive Eu anomaly (Eu/Eu*) may be due to the absence of plagioclase, diagenesis processes, hydrothermal activities, or biological productivity, while the negative Ce anomaly (Ce/Ce*) could be attributed to the formation of a soluble organic compound (Ce⁴⁺) under reductive conditions (Trubelja et al., 1995; Kurian et al., 2008; Loges et al., 2012; Abedini and Calagari, 2015). As clastic levels rise, the Ce/Ce* ratio approaches one (Madhavaraju and González-León, 2012). This ratio ranges between < 0.1 and 0.4 in seawater (Piepgras and Jacobsen, 1992). The Ce/Ce* ratios in the studied sediments range from 0.69 to 0.94 (average = 0.79) (Table 2). This indicates that the sediments were deposited from seawater with a slight detrital influx from neighboring continental sediments.

Figure 8. Chondrite-normalized REE pattern of sediments across the K/Pg boundary for the Bade section, Dohuk area, northern Iraq. Chondrite values are taken from Sun and McDonugh (1989); PAAS and UCC values are taken from Taylor and McLennan (1985).

The proposed K/Pg boundary (sample B20 “goethite layer”) shows the lowest total rare earth elements (ΣREE) compared to other samples. The lowest ΣREE could be attributed to low REE contents in carbonaceous-type asteroid which is likely Chicxulub impactor (Smit and ten Kate, 1982; Lodders et al., 2009; Fischer-Gödde et al., 2024) and/or low environmental pH, which enhances REE movement and depletion (Staudigel and Hart, 1983). Moreover, the PGEs anomaly proves the K/Pg boundary (Alvarez et al., 1980; Claeys et al., 2002). Unfortunately, PGEs were undetected (< 5 pbb) in five samples (B18-B22) across the studied K/Pg boundary. It suggests that the PGEs, if they exist, may be undetectable at concentrations below 5 ppb in Iraqi K/Pg boundary sites.

At the goethite layer of this study, Fe₂O₃, TTEs, Zr, Ta, Th, U, Ba, Ga, Pb, As, Bi, Cd, In, Mo, S, Sb, Te, W, Zn, and Cu show positive anomalies, while SiO₂, Al₂O₃, MgO, CaO, Na₂O, K₂O, TiO₂, P₂O₅, MnO, LOI, Hf, Nb, Y, Rb, Sr, Cs, Li, Ag, Be, Sn, Tl, LREEs, and HREEs d isplay negative anomalies (Figs. 9, 10). The K/Pg boundary layers around the world often contain significant concentrations of Fe₂O₃, Ni, Co, Cr, Sc, V, Zr, Th, U, Pb, As, S, Sb, Zn, and Cu (Kyte et al., 1980; Alvarez et al., 1980; Smit and ten Kate, 1982; Schmitz, 1988; Vajda and Wigforss-Lange, 2006). Therefore, proponents of the Chicxulub impact theory propose the asteroid and target zone as sources for these elements (Smit and ten Kate, 1982; Gilmour and Anders, 1989). On the other hand, supporters of the Deccan volcanic theory argue that these elements were derived from the Deccan cloud's post-fallout (Crocket et al., 1988; Graup et al., 1989). The Chicxulub and Deccan clouds could produce an atmospheric acidity condition due to the significant release of SO₂ (Officer and Drake, 1985; Prinn and Fegley, 1987). This scenario leads to a higher SO₂ content and SO₂/MnO ratio at the K/Pg boundary layer (Rampino, 2010; Keller, 2014; Renne et al., 2015; Richards et al., 2015). This study reveals that the K/Pg boundary (sample B20 “goethite layer”) shows the highest levels of SO₂ and SO₂/MnO ratio (Table 3), indicating severe acidic rainfall from Chicxulub and/or Deccan clouds during the K/Pg event. Moreover, a sharp reduction in CaO may be attributed to acidic rain caused by Chicxulub and/or Deccan events, which temporarily prevented the development of biogenic CaCO₃ during the K/ Pg event (Barash, 2011; Keller et al., 2011).

Table 3 . SO₂ content (in wt %), SO₂/MnO ratio, and variations of paleoredox elements across the K/Pg boundary for the Bade section, Dohuk area, northern Iraq. U_EF = [(U/Al)_sample/(U/Al)_PAAS] and Mo_EF = [ (Mo/Al)_sample/(Mo/Al)_PAAS] are from Tribovillard et al. (2012); PAAS values are taken from Taylor and McLennan (1985).

Sample	SO2	SO2/MnO	Fe/Al	Ni/Al	U/Al	Pb/Al	Cr/Al	V/Al	Cu/Al	Zn/Al	Mo/Al	U/Mo	UEF	MoEF
B23	0.24	8.20	0.43	0.0091	0.0002	0.0001	0.0039	0.0032	0.0011	0.0054	0.0001	1.78	8.00	13.95
B22	0.07	2.11	1.13	0.0082	0.0003	0.0001	0.0040	0.0035	0.0010	0.0058	0.0002	1.82	9.17	15.66
B21	0.06	1.47	1.17	0.0097	0.0001	0.0001	0.0046	0.0036	0.0011	0.0070	0.0002	0.51	2.90	17.58
B20 (K/Pg)	0.25	11.72	68.15	0.1749	0.0004	0.0158	0.0409	0.0662	0.0353	0.1013	0.0070	0.06	13.62	699.54
B19	0.07	1.97	0.89	0.0069	0.0001	0.0001	0.0032	0.0025	0.0010	0.0021	0.0001	0.77	3.30	13.28
B18	0.28	9.14	0.92	0.0103	0.0002	0.0014	0.0053	0.0031	0.0012	0.0026	0.0001	1.59	6.47	12.59
B17	0.14	4.63	1.08	0.0092	0.0003	0.0004	0.0052	0.0039	0.0014	0.0045	0.0002	1.99	10.39	16.22

Figure 9. Vertical distribution profile plot across the K/Pg boundary for the Bade section, Dohuk area, northern Iraq. (A) Major oxides and loss on ignition (LOI). (B) Transition trace elements (TTEs). (C) High field strength elements (HFSEs). The gray area represents the K/Pg boundary layer (goethite layer).

Figure 10. Vertical distribution profile plot across the K/Pg boundary for the Bade section, Dohuk area, northern Iraq. (A) Large ion lithophile elements (LILEs). (B) Ag, As, Be, Bi, Cd, In, Mo, S, Sb, Sn, Te, Tl, W, Zn, and Cu elements. (C) Light rare earth elements (LREEs) and heavy rare earth elements (HREEs). The gray area represents the K/Pg boundary layer (goethite layer).

We believe that the high concentrations of Pb, As, S, Sb, Zn, and Cu trace elements at the goethite layer (Fig. 10 A, B) support the idea that the goethite phase is weathered products from pyrite diagenesis, which could explain the low concentrations of SiO₂, Al₂O₃, MgO, CaO, Na₂O, K₂O, TiO₂, P₂O₅, and MnO at the goethite l ayer (Fig. 9 A). On the other hand, the reasons behind the depletion of trace elements at low K/Pg temperatures remain unclear. The depletion mechanisms are influenced by trace element solubility, oxygen fugacity, and pH in aqueous environments. Consequently, we are simply describing the possibility of detecting the K/Pg boundary and the final change to goethite phase, a process that causes a depletion of Hf, Nb, Y, Rb, Sr, Cs, Li, Ag, Be, Sn, and REE (Ritter et a l., 2015).

Al-normalized authigenic redox-sensitive element concentrations (Fe/Al, Ni/Al, U/Al, Pb/Al, Cr/Al, V/Al, Cu/Al, Zn/Al, and Mo/Al), enrichment factors of U and Mo (U_EF and Mo_EF), and U/Mo ratio support the significant geochemical anomalies at the studied goethite layer (K/ Pg). The sudden increase of Al-normalized elements, U_EF, and Mo_EF at the studied goethite layer (Table 3; Fig. 11) indicates a change in redox conditions just within this boundary layer compared to the Upper Cretaceous and Lower Paleogene sediments, which means a rapid return to the oxygen conditions of the Upper Cretaceous after the K/Pg event. Moreover, the high levels of U and Mo are good paleoredox indicators (Algeo and Tribovillard, 2009; Tribovillard et al., 2012). U and Mo enrichments in marine deposits are often due to their authigenic addition from seawater under suboxic and euxinic conditions (for U and Mo, respectively) (Sosa-Montes de Oca et al., 2013). Therefore, a significant drop in the U/Mo ratio at the studied goethite layer (sample B20) (Table 3; Fig. 11) refers to sulfidiceuxinic conditions caused by extreme acidic rainfall from Chicxulub and/or Deccan clouds. This explains how pyrite formed under these conditions and developed into goethite in a fluid-rich oxidized environment that may have been restored over time after the K/Pg event.

Figure 11. Paleoredox profile versus stratigraphic height across the K/Pg boundary for the Bade section, Dohuk area, northern Iraq, showing a change in redox conditions just within the goethite layer compared to the Upper Cretaceous and Lower Paleogene sediments. The gray area represents the K/Pg boundary layer (goethite layer).

The field observations showed that the proposed K/Pg layer lies within the marl layers and consists of an iron layer composed of black spherules and matrix (iron spherulerich layer).

The petrographic study suggested that sedimentation just below and above the proposed K/Pg boundary occurred in calm, low-energy water, which preserved the iron spherule-rich layer in the sediments.

The XRD results revealed that the Upper Cretaceous and Lower Paleogene layers were composed of calcite, quartz, muscovite, and anatase phases, while the proposed K/Pg boundary (spherules and matrix) was predominantly of goethite and a trace amount of barite, giving it the name goethite layer. Goethite dominance and a sharp drop in calcite inside the goethite layer indicated that this layer marks the K/Pg boundary.

The geochemical results indicated that the PGEs may be undetectable at concentrations below 5 ppb in Iraqi K/Pg boundary sites. The REE pattern revealed that the Bade sediments shared the provenance, and the Ce/Ce* ratios (0.69-0.95) proposed they were deposited from seawater with a slight detrital influx from neighboring continental sediments. Moreover, the positive Eu/Eu* was mostly attributed to the absence of plagioclase in the Bade sediments. The elevated levels of Fe₂O₃, Ni, Co, Cr, Sc, V, Zr, Th, U, Pb, As, S, Sb, Zn, and Cu with CaO depletion within the goethite layer (sample B20) suggested that this layer represents the K/Pg boundary. Additionally, the lowest total REE (ΣREE) content and the highest levels of SO₂ and SO₂/MnO at the goethite layer attested to its boundary status. The sudden increase of authigenic redox-sensitive elements (element/Al ratios), U_EF, and Mo_EF indicated a change in redox conditions just within the goethite layer compared to the Upper Cretaceous and Lower Paleogene sediments, which means a rapid return to the oxygen conditions of the Upper Cretaceous after the K/Pg event. Furthermore, the sharp decline in the U/Mo ratio at the goethite layer reflected the sulfidic-euxinic conditions that led to the development of pyrite.

Based on these conclusions, the goethite layer (sample B20) represents the K/Pg boundary layer, and the Bade section is considered the perfect K/Pg boundary section in Iraq.

The authors express gratitude to the Geology Department at Basrah University for providing facilities, Dr. Abdulrahman Bamerni for fieldwork assistance and his valuable discussions that supported the preparation of this paper, Dr. Kotayba Al-Youzbakey for his help with mineralogical diagnosis, and Dr. Zaid Malak for his assistance with the petrographic study.